CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2005-021991, filed Jan. 28, 2005, the contents of which are hereby incorporated by reference into the present application. 
   TECHNICAL FIELD 
   The disclosure relates to an image forming device such as a laser printer, and a process cartridge and a developer cartridge which are, in use, mounted on the image forming device. 
   BACKGROUND 
   Conventional image forming devices such as a laser printer use a photosensitive cartridge in which a photosensitive drum is rotatably supported. A developer cartridge is also used therein for supplying the photosensitive drum with toner. The photosensitive cartridge and the developer cartridge are mountable to and detachable from the main body of the device. Only necessary one or ones of the cartridges can be replaced with a new one when the lifetime of the cartridge is ended. 
   Japanese Patent Application Publication No. 2000-267547 discloses a photosensitive cartridge, for use in an image forming device. The photosensitive cartridge has a flat lower surface facilitating to place the cartridge removed from the image forming device on a flat table. 
   With the photosensitive cartridge according to the proposal described above, the cartridge can be placed stably on a flat plane. However, the shape of the upper surface of the cartridge does not allow another photosensitive cartridge to be stacked thereon. Typically, a manufacturer collects and stores, in a stock room, old photosensitive cartridges or developer cartridges detached from the main body of the device for recycling In this case, it is convenient if the cartridges can be stacked one on the other. Otherwise, inconvenience is caused in handling the old cartridges and a large space is required for storage. 
   SUMMARY 
   In view of the foregoing, it is an object of the present invention to provide a process cartridge and a developer cartridge which can be stacked stably, without packing the cartridges in boxes, and an image forming device including such a process cartridge or developer cartridge. 
   In order to attain the above and other objects, the present invention provides a process cartridge being detachably mountable in an image-forming device. The process cartridge includes a main casing. The main casing includes a first wall, a second wall, and a third wall. The first wall is formed with at least one first engagement part. The second wall is formed with at least one second engagement part. The second wall is disposed in confronting relation with the first wall. The third wall connects the first wall and the second wall. When a plurality of the process cartridges are stacked one on the other with the first wall being downside with respect to the second wall, the first engagement part in one process cartridge engages the second engagement part in another process cartridge disposed just below the one process cartridge. 
   According to another aspect of the present invention provides a developer cartridge, detachably mountable in an image-bearing member cartridge. The developer cartridge includes a developer main casing. The developer main casing includes a first developer wall and a second developer wall. The first developer wall is formed with a convex part. The second developer wall is formed with an insertion portion inserted by the convex part when the developer cartridge is mounted on the image-bearing member cartridge. The second developer wall is disposed in confronting relation with the first developer wall. 
   According to another aspect of the present invention provides a developer cartridge being detachably mountable in an image-bearing member cartridge. The developer cartridge includes a developer main casing. The developer main casing includes a first developer wall and a second developer wall. The first developer wall is formed with a first developer engagement part. The second developer wall is formed with a second developer engagement part. The second developer wall is disposed in confronting relation with the first developer wall. When a plurality of the developer cartridges are stacked one on the other with the first developer wall being downside with respect to the second developer wall facing upward, the first developer engagement part in one process cartridge engages the second developer engagement part in another developer cartridge disposed just below the one developer cartridge. 
   According to another aspect, of the present invention provides an image-forming device. The image-forming device includes a process cartridge. The process cartridge includes a main casing. The main casing includes a first wall, a second wall, and a third wall. The first wall is formed with at least one first engagement part. The second wall is formed with at least one second engagement part. The second wall is disposed in confronting relation with the first wall. The third wall connects the first wall and the second wall. When a plurality of the process cartridges are stacked one on the other with the first wall being downside with respect to the second wall, the first engagement part in one process cartridge engages the second engagement part in another process cartridge disposed just below the one process cartridge. 
   According to another aspect of the present invention provides an image-forming device. The image-forming device includes a developer cartridge being detachably mountable in an image-bearing member cartridge. The developer cartridge includes a developer main casing. The developer main casing includes a first developer wall and a second developer wall. The first developer wall is formed with a convex part. The second developer wall is formed with an insertion portion into which the convex part is inserted when the developer cartridge is mounted on the image-bearing member cartridge. The second developer wall is disposed in confronting relation with the first developer wall. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a side cross-sectional view of a main part of a laser printer according to a preferred embodiment of the present invention; 
       FIG. 2  is a side cross-sectional view of a process cartridge shown in  FIG. 1 ; 
       FIG. 3  is a plan view of a drum cartridge shown in  FIG. 1 ; 
       FIG. 4  is a bottom view of the drum cartridge shown in  FIG. 1 ; 
       FIG. 5  is a side view of the drum cartridge shown in  FIG. 1 ; 
       FIG. 6  is a plan view of a developer cartridge shown in  FIG. 1 ; 
       FIG. 7  is a bottom view of the developer cartridge shown in  FIG. 1 ; 
       FIG. 8  is a side view of the developer cartridge shown in  FIG. 1 ; 
       FIG. 9  is a side view of the process cartridge shown in  FIG. 1  when the developer cartridge is mounted on the drum cartridge; 
       FIG. 10  is a bottom view of the process cartridge shown in  FIG. 1 , when the developer cartridge is mounted on the drum cartridge; 
       FIG. 11  is a side view of the process cartridge shown in  FIG. 1 , when the drum cartridges are stacked; 
       FIG. 12  is a side view of stacked process cartridges each shown in  FIG. 1 , when each developer cartridge is mounted on the drum cartridge; and 
       FIG. 13  is a side view of stacked developer cartridges each shown in  FIG. 1 . 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the present invention will be described while referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals to avoid duplicating description. 
   &lt;Overall Structure of Laser Printer&gt; 
     FIG. 1  is a side cross-sectional view showing a laser printer as an image forming device according to an embodiment of the present invention. In  FIG. 1 , the laser printer  1  has a feeder section  4  and an image forming section  5 , in a body casing  2 . The feeder section  4  serves to feed a paper sheet  3  as a recording medium. The image forming section  5  serves to form an image on the paper sheet  3 . 
   &lt;Structure of Body Casing  2 &gt; 
   A sheet discharge tray  6  for receiving the paper sheet  3  on which an image has been formed is formed on the upper surface of the body casing  2 . On one side of the sheet discharge tray  6 , an operation panel having operation keys, and an LED display portion are provided. Further, an opening  7  through which a process cartridge  22  to be described in detail is detachably mounted is formed on the side wall at the operation panel side in the body casing  2 . A front cover  8  is provided for opening and closing the opening  7 . The front cover  8  is pivotably movably supported by a cover shaft (not shown) inserted in a lower end portion of the front cover  8 . The front cover  8  is pivotably moved to selectively open and close the opening. Specifically when the front cover  8  is opened (tilted), the opening  7  is opened whereas when the front cover  8  is closed, the opening  7  is closed. Through the opening  7 , the process cartridge  22  can be mounted on and detached from the body casing  2 . 
   In the description below, the side in which the front cover is provided is defined as the “front side” of this laser printer  1 , and the opposite side is as the “rear side”. The direction perpendicular to sheet of drawing in  FIG. 1  is defined as the widthwise direction of the laser printer  1 . 
   &lt;Structure of Feeder Section&gt; 
   The feeder section  4  includes a sheet feed tray  9 , a sheet press plate  10 , a sheet feed roller  11 , a sheet feed pad  12 , paper powder removal rollers  13  and  14 , and registration rollers  15 . The sheet feed tray  9  is detachably mounted on a bottom portion of the body casing  2 . The sheet press plate  10  is provided in the sheet feed tray  9 . The sheet feed roller  11  and sheet feed pad  12  are provided in the front and upper side of the sheet feed tray  9 . The paper powder removal roller  13  is disposed in opposition to the sheet feed roller  11  with the paper sheet conveying path intervened therebetween. Another paper powder removal rollers  14  are disposed in the downstream side of the sheet feed roller  11  in the conveying direction of a paper sheet  3 . The registration rollers  15  are disposed in the downstream side of the paper powder removal rollers  13  and  14  in the conveying direction of the paper sheet  3 . 
   The sheet press plate  10  adapted to receive paper sheets  3  to be stacked thereon. When the paper sheets  3  are stacked on the sheet press plate  10 , an end portion of the sheet press plate  10  moves away from the sheet feed roller  11 . The sheet press plate  10  is urged upwardly by a spring (not shown) from the back side thereof. Therefore, as the number of stacked paper sheets  3  increases, the sheet press plate  10  is moved downwardly against the pressing force of the spring. The sheet feed roller  11  is disposed in confronting relation with the sheet feed pad  12 . The sheet feed pad  12  is pressed against the sheet feed roller  11  by a spring  16  provided on the back side of sheet feed pad  12 . 
   The paper sheet  3  stacked at the uppermost position on the sheet press plate  10  is pressed against the sheet feed roller  11  by the pressing force of the spring from the back side of the sheet press plate  10 . The uppermost paper sheet  3  is sandwiched between the sheet feed roller  11  and the sheet feed pad  12  and fed by rotation of the sheet feed roller  11  upon being separated from the remaining paper sheets  3 . 
   Further, paper powder is removed by the paper powder removal rollers  13  and  14 . Thereafter, the paper sheet  3  is conveyed by the registration rollers  15 . 
   The registration rollers  15  consist of a pair of rollers opposed to each other. After registration of the paper sheet  3  is completed, the registration rollers  15  convey the paper sheet  3  to a transfer position between a photosensitive drum  32  and a transfer roller  34  where a toner image on the photosensitive drum  32  is transferred to the paper sheet  3 . 
   The feeder section  4  further includes a multipurpose tray  17 , a multipurpose tray side sheet feed roller  18 , and a multipurpose tray side sheet feed pad  19 . The multipurpose tray side sheet feed roller  18  and multipurpose tray side sheet feed pad  19  serve to feed paper sheets  3  stacked on the multipurpose tray  17 . The multipurpose tray side sheet feed roller  18  and the multipurpose tray side sheet feed pad  19  are disposed in opposition to each other. The multipurpose tray side sheet feed pad  19  is pressed toward the multipurpose tray side sheet feed roller  18  by a spring  20  provided in the back side of the multipurpose tray side sheet feed pad  19 . 
   The paper sheets  3  stacked on the multipurpose tray  17  are separated one after another and fed into a nip between the multipurpose tray side sheet feed roller  18  and the multipurpose tray side sheet feed pad  19 . The paper sheet  3  is fed by rotation of the multipurpose tray side sheet feed roller  18 . 
   &lt;Structure of Image Forming Section&gt; 
   The image forming section  5  includes a scanner section  21 , a process cartridge  22 , a fixing section  23 . 
   &lt;Structure of Scanner Section&gt; 
   The scanner section  21  is disposed at an upper portion inside the body casing  2 . The scanner section  21  includes a laser light emission section (not shown), a polygon mirror  24 , lenses  25  and  26 , and reflection mirrors  27 ,  28 , and  29 . A laser beam modulated based on image data emitted from the laser light emission section. Then the modulated laser beam is reflected by the polygon mirror  24 , passes through the lens  25 , and reflected by or passes through reflection mirrors  27  and  28 , lens  26 , and reflection mirror  29  in this order, as indicated by a chain line in  FIG. 1 . The laser beam is thus irradiated onto the surface of the photosensitive drum  32  contained in the process cartridge  22 . 
   &lt;Structure of Process Cartridge&gt; 
     FIG. 2  is a side cross-sectional view of the process cartridge  22 . 
   The process cartridge  22  is detachably mounted on the body casing  2  and disposed below the scanner section  21 . This process cartridge  22  includes a drum cartridge  30 , and a developer cartridge  31  which is detachably mounted on the drum cartridge  30 . 
   &lt;Structure of Drum Cartridge&gt; 
   As shown in  FIG. 2 , the drum cartridge  30  includes a cartridge frame  103 , a photosensitive drum  32  disposed in the cartridge frame  103 , a scorotron charger  33 , a transfer roller  34 , and a cleaning brush  35 . 
     FIG. 3  is a plan view of a drum cartridge  30 .  FIG. 4  is a bottom view of the drum cartridge  30 .  FIG. 5  is a side view of the drum cartridge  30 . 
   As shown in  FIG. 3 , a cartridge frame  103  integrally has a left side wall  36 , a right side wall  37 , a bottom wall  38 , a front wall  39 , and a rear upper wall  40 . 
   The left side wall  36  and the right side wall  37  are opposed to each other with an interval in the widthwise direction therebetween. Further, the left side wall  36  and the right side wall  37  have substantially symmetrical structures in the widthwise direction. As shown in  FIG. 5 , each of the left side wall  36  and right side wall  37  has a rear side wall portion  41  having a substantially bow-side like shape when viewed from a side, and a front side wall portion  42  extending frontwardly from the rear side wall portion  41 . 
   In the front side wall portion  42 , a roller shaft guide portion  43 , and a roller shaft receiving portion  44  are formed. The roller shaft guide portion  43  guides a shaft end portion of a developer roller shaft  91  described later when the developer cartridge  31  is attached to the drum cartridge  30 . The roller shaft receiving portion  44  is formed continuously with a rear end of the roller shaft guide portion  43 , and receives the end portion of the developer roller shaft  91  guided by the roller shaft guide portion  43 . 
   The roller shaft guide portion  43  is formed as a part of an upper end edge of the front side wall portion  42 . That is, the roller shaft guide portion  43  extends obliquely downwardly toward the rear side from the middle of the front side wall portion  42 . The roller shaft guide portion  43  is downwardly shaped and gradually flattened toward the rear side. 
   The roller shaft receiving portion  44  is formed below a protruding wall  45  and continuous to the rear side of the roller shaft guide portion  43 . The roller shaft receiving portion  44  is a substantially in a rectangular shape when viewed from a side. A lower end edge of the protruding wall  45  is formed continuously with the rear end edge of the roller shaft guide portion  43 . 
   Also, the front side wall portion  42  has a front engagement convex portion  46 . When plural drum cartridges  30  are stacked one on the other, the front engagement convex portion  46  of the lower drum cartridge  30  is engageable with a front engagement concave portion  49  of the upper drum cartridge  30 , described later This front engagement convex portion  46  is flat at the top and formed as a part of the upper end edge of the front side wall portion  42 . Further, the upper front end portion  47  of the front side wall portion  42  extends obliquely downwardly toward the front side, the front engagement convex portion  46  is higher than the top flat portion of the front end portion  47  and the roller shaft guide portion  43 . 
   As shown in  FIG. 3 , the bottom wall  38  is substantially flat in shape, and is provided to connect lower end edges of the left side wall  36  and the right side wall  37  in the front-to-rear direction. As shown in  FIGS. 4 and 5 , the bottom wall  38  has a pair of rear engagement convex portions  48 , at a position below the drum shaft  56  of a photosensitive drum  32  and right and left side end portions. When plural drum cartridges  30  are stacked one on the other, the rear engagement convex portions  48  are engaged with (inserted in) rear engagement concave portions  52  (described later) of an upper drum cartridge  30 . Each of the rear engagement convex portions  48  is formed of a thin plate which is curved to protrude downwardly. When a drum cartridge  30  is put on a flat mount surface S, each of the rear engagement convex portions  48  contacts the mount surface S, and supports the drum cartridge  30  such that the rear upper wall  40  and the front engagement convex portion  46  are substantially parallel to the mount surface S. 
   Also, a pair of front engagement concave portions  49  is formed in the bottom wall  38  at positions opposed in the widthwise direction. When plural drum cartridges  30  are stacked one on the other, the front engagement concave portions  49  are engaged with (receive) the front engagement convex portions  46  of an upper drum cartridge  30 . The front engagement concave portion  49  is substantially in a rectangular shape when viewed from the bottom. 
   Further, as shown in  FIG. 4 , a pair of insertion portions  62  is formed at the left and right side end portions of the bottom wall  38 . When a plurality of the drum cartridge  30  is stacked one on the other, the developer engagement convex portion  80 , described later, of the lower drum cartridge  30  can be inserted in the insertion portion  62  of upper drum cartridge  30 . Each of the insertion portions  62  is provided at a position near the center with respect to the front-to-rear direction, ie., between the rear engagement convex portion  48  and the front engagement concave portion  49 . The insertion portions  62  are in the form of a through-hole having a substantially rectangular shape when viewed from the bottom. 
   The front wall  39  is bent upwardly from the front end edge of the bottom wall  38 . This front wall  39  is substantially in a rectangular shape. Both end portions of the front wall  39  in the widthwise direction are bent perpendicularly and are formed continuously with the left side wall  36  and right side wall  37 . 
   As shown in  FIGS. 3 and 5 , the rear upper wall  40  is a flat, plate-like member and is provided so as to connect to the upper end edges of the rear side wall portions  41  of the left side wall  36  and the right side wall  37 . At the front portion of the rear upper wall  40 , a laser input window  50  is formed which is substantially rectangular in shape when viewed from the top extends in the widthwise direction, as shown in  FIG. 3 . The rear upper wall  40  has a charger support portion  51  for supporting a Scorotron charger  33 , which is disposed behind the laser input window  50 . 
   Further, in the rear upper wall  40 , a pair of rear engagement concave portions  52  is formed. When plural drum cartridges  30  are stacked one on the other, the pair of rear engagement concave portions  52  of the lower drum cartridge  30  is engageable with the pair of the rear engagement convex portions  48  at the left and right side end portions of the rear upper wall  40  of the upper drum cartridge  30 . Each of the rear engagement concave portions  52  is provided at a position opposed in the vertical direction to the rear engagement convex portions  48 . 
   Further, in the cartridge frame  103 , a drum accommodating section  53  which accommodates the photosensitive drum  32 , is formed by the rear side wall portions  41  of the left and right side walls  36  and  37 , the rear upper wall  40 , and the rear portion of the bottom wall  38  which is opposed in the vertical direction to the rear upper wall  40 . The drum accommodating section  53  is open to the front. A developer cartridge accommodating section  54  which accommodates the developer cartridge  31  is formed by the front side wall portions  42  of the left and right side walls  36  and  37 , and the front portion of the bottom wall  38  formed continuously with each of the front side wall portions  42  in the widthwise direction. The developer cartridge accommodating section  54  is open in the upper side and communicates with the drum accommodating section  53  in the rear side. 
   As shown in  FIG. 2 , the photosensitive drum  32  has a cylindrical drum body  55  and a metal-made drum shaft  56 . The drum body  55  is formed of a photosensitive layer having positive charges. A surface layer of the drum body  55  is made of polycarbonate. The drum shaft  56  extends in the longitudinal direction of the drum body  55  through the center of the drum body  55  to be loosely rotatable about the drum body  55 . The drum shaft  56  is fixedly supported by the left and right side walls  36  and  37  of the drum cartridge  30 . Thus, the photosensitive drum  32 , disposed between the left and the right side walls  36  and  37  is rotatable around the drum shaft  56 . 
   The Scorotron charger  33  is disposed above the photosensitive drum  32  and supported by the charger support portion  51 . The Scorotron charger  33  is disposed opposite to the photosensitive drum  32  without contacting each other. A predetermined interval is maintained between the Scorotron charger  33  and the photosensitive drum  32 . This Scorotron charger  33  has a wire  57 , grid  58 , and a wire cleaner  59 . 
   The wire  57  is stretched between the left and right side walls  36  and  37  while imparting a predetermined tension therebetween. 
   The grid  58  extends in the widthwise direction to surround the lower side of the wire  57 . The grid  58  is bridged between the left and right side walls  36  and  37 . 
   A wire cleaner  59  (see  FIG. 3 ) is provided to be slidably movable in the widthwise direction of the charger support portion  51 , while sandwiching and contacting the wire  57 . The sliding movement of the wire cleaner  59  cleans the wire  57 . 
   A transfer roller  34  is rotatably supported between the left and right side walls  36  and  37 . As shown in  FIG. 2 , the transfer roller  34  is opposed to and contacts the photosensitive drum  32 , thereby forming a nip between the transfer roller  34  and the photosensitive drum  32 . This transfer roller  34  includes a transfer roller shaft  60  made of metal, and a roller  61  made of a conductive rubber material. 
   A cleaning brush  35  is disposed at the rear side of the photosensitive drum  32 . A lot of bristles of the cleaning brush  35  are supported on a support plate having an elongated rectangular shape extending in the widthwise direction. The cleaning brush  35  is opposed to the photosensitive drum  32  in the front-to-rear direction, such that the bristles contact the surface of the photosensitive drum  32  along the widthwise direction. 
   &lt;Structure of Developer Cartridge&gt; 
     FIG. 6  is a plan view of the developer cartridge  31 .  FIG. 7  is a bottom view of the developer cartridge  31 .  FIG. 8  is a side view of the developer cartridge  31 . 
   The developer cartridge  31  is detachably mounted in the developer cartridge accommodating section  54 . As shown in  FIG. 2 , the developer cartridge  31  includes a box-like developer casing  63  open in the rear side, a feed roller  64 , a developer roller  65 , and a layer-thickness regulation blade  66 . 
   As shown in  FIGS. 6 and 7 , the developer casing  63  is defined by a left side wall  67 , a right side wall  68 , a lower wall  69 , and an upper wall  70 . The left side wall  67  and the right side wall  68  are disposed opposed to each other, with an interval therebetween in the widthwise direction. The lower wall  69  and the upper wall  70  are connected to the left and right side walls  67  and  68 . As shown in  FIG. 9 , when the developer cartridge  31  is mounted in the developer cartridge accommodating section  54  of the drum cartridge  30  and the drum cartridge  30  on which the developer cartridge  31  is mounted is put on the mount surface S, the upper surface (upper wall  70 ) is held substantially at the same height as the upper surface of the rear upper wall  40 . 
   The left and right side walls  67  and  68  are in a plate-like shape extending in the front-to-rear direction. The upper wall  70  is bridged between upper end edges of both walls. The left and right side walls  67  and  68  sandwich the lower wall  69 , and are provided such that inner surfaces of the walls  67  and  68  are opposed to each other. 
   As shown in  FIG. 8 , the left side wall  67  is provided with an intermediate gear  72 , an agitator drive gear  73 , a developer roller drive gear  74 , and a feed roller drive gear  75 . The intermediate gear  72  is toothed with an input gear  71 . The agitator drive gear  73  is provided in the front side of the intermediate gear  72 , and is meshingly engaged with the intermediate gear  72 . The developer roller drive gear  74  is positioned obliquely below the input gear  71  in the rear side of the input gear  71 , and is meshingly engaged with the input gear  71 . The feed roller drive gear  75  is provided below the input gear  71  and is meshingly engaged with the input gear  71 . Drive force from a motor (not shown) is applied to the input gear  71 . 
   As shown in  FIG. 8 , a toner filling port  76  for filling toner in a toner accommodating section  85  is formed in the left side wall  67  obliquely above the agitator drive gear  73 . The toner filling port  76  is circular in shape, penetrating the left side wall  67  in the thickness direction at the position corresponding to the toner accommodating section  85 . The toner filling port  76  is closed by a cap  77  for preventing toner in the toner accommodating section  85  from leaking out of the toner filling port  76 . 
   The lower wall  69  is a plate-like member extending in the front-to-rear direction and the widthwise direction (see  FIG. 7 ), and includes a rear lower wall portion  78  and a front lower wall portion  79 . The rear lower wall portion  78  serves for partitioning a developer room  84  described later. The front lower wall portion  79  is continuous to the front end edge of the rear lower wall portion  78 , and has a substantially arcuate cross-sectional shape along the rotation orbit of an agitator  87  described later. The lower wall  69  is held between the left side wall  67  and the right side wall  68 . 
   At each of the left and right end side portions of the rear lower wall portion  78  (both side end portions in the widthwise direction), a developer engagement convex portion  80  is provided. The developer engagement convex portion  80  is inserted in insertion portions  62  of the drum cartridge  30  when the developer cartridge  31  is mounted in the drum cartridge  30 . Each of the developer engagement convex portions  80  is in a substantially rectangular shape when viewed from a side. Each of the developer engagement convex portions  80  is provided outside an area through which a paper sheet  3  entering between the photosensitive drum  32  and the transfer roller  34  passes. As shown in  FIG. 8 , when the developer cartridge  31  is put on the flat mount surface S, the developer engagement convex portions  80  contact the mount surface S and support the developer cartridge  31  such that the upper wall  70  is substantially parallel to the mount surface S. 
   As shown in  FIG. 2 , a lower partition portion  81  having a substantially triangular cross-sectional shape and protruding upwardly is formed along the widthwise direction, at the boundary between the rear lower wall portion  78  and the front lower wall portion  79 . 
   As shown in  FIG. 6 , the upper wall  70  is a plate-like member and is bridged between upper end edges of the left and right side walls  67  and  68 . At the rear end portions of the upper wall  70 , developer engagement concave portions  82  are formed at the left and right side end portions of the upper wall  70 . When plural developer cartridges  31  are stacked one on the other, the developer engagement concave portions  82  of the lower developer cartridge are engageable with the developer engagement convex portions  80  of the upper developer cartridge  31 . The developer engagement concave portion  82  in the left side is substantially rectangular in shape when viewed from the top. The concave portion  82  in left side is engaged with the lower end portion of the left developer engagement convex portion  80  of an upper developer cartridge  31  when plural developer cartridges  31  are stacked. On the other side, the right developer engagement concave portion  82  is formed in the form of a stepped portion lower in level by one step than the upper surface of the upper wall  70 . The right side concave portion  82  is also engaged with the lower end portion of the right developer engagement convex portion  80  of the upper developer cartridge  31  when plural developer cartridges  31  are stacked. 
   As shown in  FIG. 2 , an upper partition plate  83  protruding downwardly is formed along the widthwise direction on the lower surface of the upper wall  70 , opposed to the lower partition portion  81  of the lower wall  69 . 
   Further, in this developer casing  63 , an inner space at the front side from the upper and lower partition portions  83  and  81 , is partitioned and formed as developer room  84 . Another inner space at the rear side is partitioned and formed as a toner accommodating section  85 . 
   In the toner accommodating section  85 , toner made from electrically positive, non-magnetic component is accommodated as a developer. Used as the toner is polymerized toner which is obtained by a known copolymerization method by which a polymerization monomer, for example, a styrene monomer such as styrene or an acrylic monomer such as an acrylic acid, alkyl (C1 to C4) acrylate, or alkyl (C1 to C4) methacrylate are coplymerized. This kind of polymerized toner grains is spherical shape and has very excellent fluidity. Therefore, images can be formed with high image quality. 
   Toner of this kind is mixed with a coloring agent such as carbon-black, wax, or the like. In order to improve fluidity, an external additive agent such as silica is added. The grain diameter of the external additive agent is about 6 to 10 μm. 
   In the toner accommodating section  85 , an agitator  87  for stirring toner in the toner accommodating section  85  is provided. At the central portion of the toner accommodating section  85 , the agitator  87  is supported by an agitator rotation shaft  88  extending in the widthwise direction. 
   A feed roller  64  is provided in the front lower side in the developer room  84 , and is rotatably supported between the left and right side walls  67  and  68  of the developer casing  63 . This feed roller  64  is formed of a metal-made feed roller shaft  89 , and a sponge roller  90 . The feed roller shaft  89  extends in the widthwise direction. The sponge roller  90  made of an electrically conductive foaming material covers the circumference of the feed roller shaft  89 . 
   The developer roller  65  is provided in the rear lower side in the developer room  84 . The developer roller  65  and the feed roller  64  are pressed against each other. The rear portion of the developer roller  65  is partially exposed rearward from the developer casing  63 . A rear part of the developer roller  65  has a metal-made developer roller shaft  91 , which is covered with a rubber roller  92  made of electrically conductive rubber material, the rubber covering the circumference of the developer roller shaft  91 . More specifically, the rubber roller  92  is made of electrically conductive urethane rubber or silicone rubber containing fine carbon grains. The surface of the rubber roller  92  is covered with urethane rubber or silicon rubber containing fluorine. As shown in  FIGS. 6 and 7 , end portions of the developer roller shaft  91  in both sides extend outwardly in the widthwise direction beyond the side plates of the developer casing  63 . 
   As shown in  FIG. 2 , the layer-thickness regulation blade  66  is formed of a metal leaf spring. The layer-thickness regulation blade  66  is provided, at the top end portion thereof with a press rubber member  93  having a semi-circular cross-section made of an electrically insulating silicone rubber. Further, the layer-thickness regulation blade  66  is supported by the developer casing  63  at a position above the developer roller  65 . The lower end portion of the layer-thickness regulation blade  66  is in contact with the rubber roller  92  of the developer roller  65 . The press rubber member  93  is pressed against the surface of the rubber roller  92  by elastic force of the layer-thickness regulation blade  66 . 
     FIG. 9  is a side view of a process cartridge  22  in which the developer cartridge  31  is mounted on the drum cartridge  30 .  FIG. 10  is a bottom view of the process cartridge  22 . 
   The developer cartridge  31  is attached to the developer cartridge accommodating section  54  of the drum cartridge  30  in the following manner. That is, the developer cartridge  31  is located above the developer cartridge accommodating section  54  of the drum cartridge  30 . Further, both end portions of the developer roller shaft  91  protruding outwardly from both sides of the developer casing  63  are guided along the roller shaft guide portions  43  of the cartridge frame  103  of the drum cartridge  30 , the developer cartridge  31  is moved down. Further, both end portions of the developer roller shaft  91  are brought into contact with the rear end edges of the roller shaft receiving portions  44 , and are respectively received in the roller shaft receiving portions  44 . Then, the developer cartridge  31  is completely mounted in the drum cartridge  30 . 
   In this mounting process, the developer engagement convex portions  80  of the developer cartridge  31  are inserted in the insertion portions  62  of the drum cartridge  30 , as shown in  FIG. 10 . Therefore, each of the developer engagement convex portions  80  does not obstruct mounting the developer cartridge  31  in the drum cartridge  30 . As a result, smooth mount of the developer cartridge  31  to the drum cartridge  30  is ensured. 
   Referring to  FIGS. 2 and 8 , a drive force is applied to the input gear  71  of the developer cartridge  31 . By this drive force, the agitator  87  is rotated around the agitator rotation shaft  88 . Then, toner in the toner accommodating section  85  is stirred and expelled toward the developer room  84  through a section between the upper partition plate  83  and the lower partition portion  81 . Further, the toner supplied to the developer room  84  is conveyed onto the developer roller  65  by the rotation of the feed roller  64 . At this time, toner is frictionally positively charged when passing through a nip between the sponge roller  90  of the feed roller  64  and the rubber roller  92  of the developer roller  65 . The toner supplied onto the developer roller  65  enters a nip between the developer roller  65  and the press rubber member  93  of the layer-thickness regulation blade  66  along with the rotation of the developer roller  65 , thereby forming a thin toner layer having a constant thickness. The layer is carried on the developer roller  65 . 
   Meanwhile, the surface of the photosensitive drum  32  is positively charged uniformly by the Scorotron charger  33 . Thereafter, the surface of the photosensitive drum  32  is exposed to a laser beam from the scanner section  21 . An electrostatic latent image based on image data is formed on the photosensitive drum  32 . 
   Next, the positively charged toner carried on the developer roller  65  is supplied to the electrostatic latent image formed on the photosensitive drum  32 . That is, of the photosensitive drum uniformly charged positively, toner is attracted to the exposed parts that have been exposed to the laser beam and have a lowered potential. Accordingly, the toner is selectively carried on the photosensitive drum  32 , and the latent image is thus visualized. 
   The paper sheet  3  is fed between the photosensitive drum  32  and the transfer roller  34 . A toner image carried on the surface of the photosensitive drum  32  is transferred to the paper sheet  3 . 
   &lt;Structure of Fixing Section&gt; 
   As shown in  FIG. 1 , the fixing section  23  is provided in the rear side of the process cartridge  22  and in the downstream side in the conveying direction of the paper sheet  3 . The fixing section  23  includes a heating roller  94 , a press roller  95  which presses the heating roller  94 , and a pair of conveyer rollers  96 . The press roller  95  is opposed to the heating roller  94 . The pair of conveyer rollers  96  is provided in the downstream side of the press roller  95  in the conveying direction of the paper sheet  3 . 
   The heating roller  94  houses a halogen lamp and is made of metal for heating. In the fixing section  23 , the toner image transferred to the paper sheet  3  is thermally fixed while the paper sheet  3  passes between the heating roller  94  and the press roller  95 . Thereafter, the paper sheet  3  is conveyed to a sheet discharge path  97  by the conveyer rollers  96 . The paper sheet  3  is discharged onto the sheet discharge tray  6  by the sheet discharge rollers  98 . 
   In this laser printer  1 , residual toner remaining on the surface of the photosensitive drum  32  after transferring the toner image to the paper sheet  3  is collected by the developer roller  65 . If the toner remaining on the photosensitive drum  32  is collected by such a cleanerless method, neither a toner cleaner device nor a storage portion of waste toner are necessary. The structure of the device can thus be simplified. 
   &lt;Structure of Reverse Conveyer Section&gt; 
   The laser printer  1  is provided with a reverse conveyer section  99  to form images on both sides of the paper sheet  3 . This reverse conveyer section  99  includes sheet discharge rollers  98 , a reverse conveying path  100 , a flapper  101 , and plural reverse conveyer rollers  102 . 
   The sheet discharge rollers  98  are constituted by a pair of rollers and is constructed such that forward and reverse rotations can be switched to each other. In discharging the paper sheet  3  onto the sheet discharge tray  6 , the sheet discharge rollers  98  rotate in the forward direction. Otherwise, in reversing the paper sheet  3 , the sheet discharge rollers  98  rotate in the reverse direction. 
   The reverse conveying path  100  is arranged along the vertical direction so that the paper sheet  3  can be conveyed from the sheet discharge rollers  98  to the plural reverse conveyer rollers  102  provided below the image forming position. An end portion of the reverse conveying path  100  in the upstream side is positioned near the sheet discharge rollers  98 . Another end thereof in the downstream side is positioned near the reverse conveyer rollers  102 . 
   The flapper  101  is pivotally disposed in a branch portion between the sheet discharge path  97  and the reverse conveying path  100 . By energization or de-energization of a solenoid (not shown), the conveying direction can be switched from the direction toward the sheet discharge path  97  to the direction toward the reverse conveying path  100 . 
   Plural reverse conveyer rollers  102  are provided in the front-to-rear direction, above the sheet feed tray  9 . The reverse conveyer roller  102  in the most upstream side is positioned near the rear end portion of the reverse conveying path  100 . The reverse conveyer rollers  102  in the most downstream side is positioned below the registration rollers  15 . 
   Further, to form images on both sides of the paper sheet  3 , this reverse conveyer section  99  is operated as follows. A paper sheet  3  having a surface on which an image has been formed is conveyed from the sheet discharge path  97  to the sheet discharge rollers  98  by the conveyer rollers  96 . Then, the sheet discharge rollers  98  forwardly rotate with the paper sheet  3  sandwiched therebetween, and convey the paper sheet  3  to the outside (the side of the sheet discharge tray  6 ). When most part of the paper sheet  3  is fed to the outside and the rear end of the paper sheet  3  is sandwiched between the sheet discharge rollers  98 , the sheet discharge rollers  98  stop rotating. Subsequently, the sheet discharge rollers  98  rotate in the reverse direction, and the flapper  101  pivots to switch the conveying direction such that the paper sheet  3  is conveyed to the reverse conveying path  100 . The paper sheet  3  is conveyed to the reverse conveying path  100  with the top and bottom of the paper sheet  3  reversed. After conveyance of the paper sheet  3  is completed, the flapper  101  is switched to an original state, i.e., a state in which the paper sheet  3  fed from the conveyer rollers  96  is sent to the sheet discharge rollers  98 . 
   Subsequently, the paper sheet  3  conveyed to the reverse conveying path  100  in the opposite direction is further conveyed to the reverse conveyer rollers  102 . From the reverse conveyer rollers  102 , the paper sheet  3  is conveyed upwardly and further reversed, and sent to the registration rollers  15 . The paper sheet  3  conveyed to the registration rollers  15  is subjected to registration again. Thereafter, the paper sheet  3  is fed to the image forming position. Images are thus formed on both sides of the paper sheet  3 . 
   &lt;Stacking of Process Cartridges&gt; 
     FIG. 11  is a side view showing a state in which two drum cartridges  30  are stacked.  FIG. 12  is a side view showing a state in which developer cartridges  31  are mounted in the drum cartridges  30 .  FIG. 13  is a side view showing a state in which two developer cartridges  31  are stacked. 
   According to the structure as described above, the rear engagement convex portions  48  are formed on the bottom wall  38  of the cartridge frame  103  of the drum cartridge  30 . Formed on the rear upper wall  40  are the rear engagement concave portions  52  engageable with the rear engagement convex portion  48 . The front engagement concave portions  49  are also formed on the bottom wall  38 . Formed on the front side wall portions  42  of the left and right side walls  36  and  37  are the front engagement convex portions  46  each being engageable with the front engagement concave portion  49 . Therefore, another drum cartridge  30  is provided on a drum cartridge  30  as shown in  FIG. 11 , or another process cartridge  22  is provided on a process cartridge  22  as shown in  FIG. 12 . Then, the rear engagement convex portions  48  of the upper drum cartridge  30  (process cartridge  22 ) can be engaged with the rear engagement concave portions  52  of the lower drum cartridge  30  (process cartridge  22 ). Simultaneously, the front engagement concave portions  49  of the upper drum cartridge  30  (process cartridge  22 ) can be engaged with the front engagement convex portions  46  of the lower drum cartridge  30  (process cartridge  22 ). Likewise, another drum cartridge  30  (process cartridge  22 ) may be stacked on the upper drum cartridge  30  (process cartridge  22 ), may be stacked on the upper drum cartridge  30  (process cartridge  22 ). Then, the rear engagement convex portions  48  and the front engagement concave portions  49  of the upper drum cartridge  30  (process cartridge  22 ) can be engaged with the rear engagement concave portion  52  and the front engagement convex portion  46  of the lower drum cartridge  30  (process cartridge  22 ) respectively. 
   As a result, plural drum cartridges  30  can be stacked stably by concave-convex engagement between the individual drum cartridges  30 . When drum cartridges  30  are detached from the laser printer  1  (body casing  2 ), the drum cartridges  30  can be handled easily. In addition, the space for storing the drum cartridge  30  can be reduced. 
   Also, plural process cartridges  22  can be stacked stably by concave-convex engagement between the individual process cartridges  22 . When process cartridges  22  are detached from the laser printer  1  (body casing  2 ), the process cartridges  22  can be handled easily. And, the space for storing the process cartridge  22  can be reduced. 
   As shown in  FIG. 4 , the rear engagement convex portions  48 , front engagement concave portions  49 , rear engagement concave portions  52 , and front engagement convex portions  46  are provided in both the left and right sides (both sides in the widthwise direction). Therefore, by concave-convex engagement between these portions, plural drum cartridges  30  can be stacked more stably. Similarly, by concave-convex engagement between these portions, plural process cartridges  22  can be stacked more stably. 
   In addition, when a drum cartridge  30  (see  FIG. 5 ) or a process cartridge  22  (see  FIG. 9 ) is put on the flat mount surface S, each of the rear engagement convex portions  48  contacts the mount surface S, and supports the drum cartridge  30  or process cartridge  22  such that the rear upper wall  40  and the front engagement convex portion  46  are substantially parallel to the mount surface S. On the drum cartridge  30  put on the mount surface S, plural drum cartridges  30  can be stacked much more stably. Similarly, on the process cartridge  22  put on the mount surface S, plural process cartridges  22  can also be stacked much more stably. 
   The rear engagement concave portions  52  and the front engagement convex portions  46  are at such positions that are opposed to the rear engagement convex portions  48  and the front engagement concave portions  49  in the vertical direction. Therefore, plural drum cartridges  30  can be stacked in the vertical direction. As a result, the plural drum cartridges  30  can be stacked more stably. Similarly, plural process cartridges  22  can be stacked in the vertical direction. As a result, the plural process cartridges can be stacked more stably. 
   Further, the rear engagement concave portions  52  and the front engagement convex portions  46  are spaced apart therebetween in the front-to-rear direction. The rear engagement convex portions  48  and the front engagement concave portions  49  are space apart therebetween in the front-to-rear direction. 
   Therefore, the state in which plural drum cartridges  30  are stacked can be kept more stably in the front-to-rear direction, by concave-convex engagement between the respective cartridges. As a result, plural drum cartridges  30  can be stacked more stably. 
   Also, the state in which plural process cartridges  22  are stacked can be kept more stably in the front-to-rear direction, by concave-convex engagement between the respective cartridges. As a result, plural process cartridges  22  can be stacked more stably. 
   In other words, the rear engagement concave portion  52  and the rear engagement convex portions  48  which are opposed to each other in the vertical direction are considered as one set. The front engagement convex portion  46  and the front engagement concave portion  49  which are opposed to each other in the vertical direction are considered as one set. These sets spaced apart in the front-to-rear direction. For each set, two sets are provided in the widthwise direction. Therefore, in the front-to-rear direction and in the widthwise direction, the plural drum cartridges  30  can be stacked one on the other, kept stable in the vertical direction. Similarly, in the front-to-rear direction and in the widthwise direction, the plural process cartridges  30  can be stacked one on the other, kept stable in the vertical direction. 
   In the present embodiment, as shown in  FIG. 9 , when the developer cartridge  31  is mounted on the developer cartridge accommodating section  54  of the drum cartridge  30 , and these cartridges are put on the mount surface S, a height from the mount surface S to the upper surface (the upper wall  70 ) of the developer casing  63  of the developer cartridge  31  is substantially equal to the height from the mount surface S to the upper surface of the rear upper wall  40 . As shown in  FIG. 12 , when the developer cartridge  31  is mounted on the drum cartridge  30 , the plurality of process cartridge  22  (the drum cartridge with the developer cartridge) can be stacked stably. 
   As shown in  FIG. 8 , the developer cartridge  31  is provided with the developer engagement convex portions  80 . When the developer cartridge  31  is put on a flat mount surface S, the developer engagement convex portions  80  contact the mount surface S and support the developer cartridge  31  such that the upper wall  70  of the developer casing  63  is substantially parallel to the mount surface S. Therefore, as shown in  FIG. 13 , another developer cartridge  31  can be stacked on a developer cartridge  31  put on the mount surface S. 
   In addition, the developer engagement convex portions  80  are provided in both the left and right sides of the developer cartridge  31 . When the developer cartridge  31  is put on a mount surface S, the upper wall  70  of the developer casing  63  substantially parallel to the mount surface S in the widthwise direction. Therefore, another developer cartridge  31  can be stacked stably on the developer cartridge  31  put on the mount surface S. 
   The insertion portions  62  are formed in the drum cartridge  30 . When the developer cartridge  31  is mounted on the drum cartridge  30 , the developer engagement convex portions  80  are inserted in the corresponding insertion portions  62 . Therefore, the developer engagement convex portions  80  are prevented from obstructing mounting of the developer cartridge  31  in the drum cartridge  30 . As a result, smooth mounting of the developer cartridge  31  to the drum cartridge  30  can be ensured. 
   Besides, each of the insertion portions  62  is provided outside the area through which the paper sheet  3  entering between the photosensitive drum  32  and the transfer roller  34  passes. Therefore, when the developer cartridge  31  is mounted on the drum cartridge  30 , the developer engagement convex portions  80  inserted in the corresponding insertion portions  62  are prevented from obstructing passage of the paper sheet  3 . 
   In addition, the developer engagement concave portions  82  each being engageable with the developer engagement convex portion  80  are formed in the upper wall  70  of the developer casing  63  of the developer cartridge  31 . Therefore, when developer cartridge  31  can be put on the developer cartridge  31 , the developer engagement convex portions  80  of the upper developer cartridge  31  can be engaged with the developer engagement concave portions  82  of the lower developer cartridge  31 . Likewise, when further developer cartridge  31  can be put on the upper developer cartridge  31 , and the developer engagement convex portions  80  of the upper developer cartridge  31  can be engaged with the developer engagement concave portions  82  of the lower developer cartridge  31 . As a result, plural developer cartridges  31  can be put, stacked stably, by engaging the developer engagement convex portions  80  with the corresponding developer engagement concave portions  82 . Therefore, when developer cartridges  31  are put out of the laser printer  1  (body casing  2 ), handling of the developer cartridges  31  can be facilitated. In addition, the space for storing the developer cartridges  31  can be reduced. 
   The developer cartridge  31  has a toner accommodating section  85  for accommodating toner. When the developer cartridge  31  is mounted on the drum cartridge  30 , the developer cartridge  31  can supply the photosensitive drum  32  supported by the drum cartridge  30  with the toner accommodated in the toner accommodating section  85 . 
   The laser printer  1  has a drum cartridge  30  (process cartridge  22 ) and a developer cartridge  31  as described above. Therefore, in case where the manufacturer of the laser printer  1  recycles process cartridges  22 , handling of drum cartridges  30  and developer cartridges  31  is facilitated. In addition, the space for storing the drum cartridges  30  and developer cartridges  31  can be reduced. 
   While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention.
A developer cartridge includes a first developer wall formed with a convex part and a second developer wall disposed in confronting relation with the first developer wall. The convex part of the first developer wall is insertable into an insertion portion of an image-bearing member cartridge when the developer cartridge is mounted on the image-bearing member cartridge. When a plurality of the developer cartridges are stacked one on the other with the first developer wall being downside with respect to the second developer wall facing upward, the first developer engagement part in one developer cartridge engages the second developer engagement part in another developer cartridge disposed just below the one developer cartridge.