CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2005-373544, filed on Dec. 26, 2005, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a vehicle seat, and more particularly to a headrest bushing into which a headrest stay is inserted for supporting a headrest of the headrest stay. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a headrest is generally installed in a vehicle seat for an automotive to improve safety of an occupant from an accident such as collisions between vehicles. This headrest possesses a height adjusting mechanism so that an arbitrary point can adjust height location by a physical constitution and preference of an occupant. Generally, two headrest stays are adhered to the headrest. The headrest stays are inserted into two headrest bushing anchored at the side of the seat back respectively and then supported. 
     In the headrest, a head impact force of an occupant seating on a seat becomes a load in the case of a collision, thus the force that is going to depress the headrest is exerted. Due to the force exerted, the headrest moves to the lower portion, an occupant&#39;s head cannot be supported, and the security of safety for the occupant is difficult because the headrest is not in condition that the occupant&#39;s head is not completely covered. Therefore, a vehicle seat with a height adjusting mechanism is provided in one of headrest bushings in support of the headrest stays (Japanese Patent Laid-Open No. 1992-193109 Official Gazette). The height adjusting mechanism permits a movement to the upper portion of the headrest, but the movement to the lower portion comes to be locked, and a button to release the lock is provided therefor. 
       FIG. 10  depicts an exploded perspective view showing the vehicle seat. The numeral  1  of  FIG. 10  depicts a seatback,  2  of  FIG. 10  depicts a headrest, and  3   a  and  3   b  are designated as headrest stays in support of a headrest  2 . The numerals  6   a  and  6   b  of  FIG. 10  are designated as headrest bushings into which a headrest stay  3  is inserted and is supported. In the headrest bushing  6   a , a height adjusting mechanism  7 , which allows the movement to the upper portion of the headrest stay  3   a  and prevent the movement to the lower portion, is installed freely in advance or retreat in a perpendicular direction to the headrest stay  3 . The height adjusting mechanism  7  is formed by an abutting lock plate  8  in contact with the headrest stay  3   a , and a spring  12  (not shown). An outside of the height adjusting mechanism  7  is covered by a housing  13 , a button  8   b  of the abutting lock plate  8  is only exposed to the outside. The headrest stay  3   a  is provided with height adjusting notch  4  in a predetermined space (hereinafter referred to as a “height adjusting notch”). The height adjusting notch  4  is formed by a horizontal surface  4   a  toward the center of the headrest stay  3   a , and an inclined surface  4   b  to increase the diameter of slowly downward from a deepest portion of the horizontal surface  4   a . In the height adjusting mechanism  7 , by pulling the headrest  2  upward an abutment  8   a  in contact with the headrest stay  3   a  of the lock plate  8  is disengaged from the height adjusting notch  4  against an elastic force of a spring  12  by action of the incline surface  4   b  of the height adjusting notch  4 . The headrest  3   a  is movable upward, and the headrest  2  can be adjusted in its height. On the contrary, the lock plate  8  is not disengaged from the height adjusting notch  4  in engagement with a horizontal surface  4   a  of the height adjusting notch  4  when the headrest  2  is pushed downward. This can prevent from moving to the lower portion of the headrest  2 , however, the button  8   b  of the lock plate  8  is disengaged from the height adjusting notch  4  by pushing against the spring  12 , thereby moving to the lower portion of the headrest stay  3   a.    
     A falling-off preventive mechanism is arranged to the headrest bushing in a free state for preventing the headrest stay from falling-off, the falling-off preventive mechanism is also provided in the inside of the seatback (U. S. patent application publication US2005/200185). 
     However, the headrest stay is inserted into the two headrest bushings, one of which is in a free state without having any of mechanisms according to the vehicle seat in Japanese Patent Laid-Open No. 1992-193109 Official Gazette. Therefore, the headrest is disengaged from the seatback easily when an upward force acts on the headrest (for example, it may be contemplated in the case that an occupant pulls the headrest out). 
     Consequently, in the other headrest bushing with the height adjusting mechanism there is provided with a vehicle seat comprising: a locking mechanism for stopping the movement to the upper portion of the headrest stay and for preventing disengagement of the headrest, and a mechanism having a locking/unlocking mechanism (hereinafter referred as to a “falling-off preventive mechanism”). 
       FIG. 11  shows a main exploded perspective view of a vehicle seat having the headrest comprising the falling-off preventive mechanism. All of the components which occur identically in  FIG. 10  have the same reference numerals respectively, a description on the components is omitted. 
     The numeral  5  of  FIG. 11  is designated as a notch for preventing the headrest  2  from falling-off (hereinafter referred as to a “falling-off preventive notch”), the numeral  9  is the falling-off preventive mechanism, and the numeral  10   b  is an operating button for unlocking the falling-off preventive mechanism  9 . The headrest bushing  6   b  is embedded in the inside of the seatback  1 , and the falling-off preventive mechanism  9  has a construction to be installed in the upper portion in the form protruded outside from the seatback  1 . The falling-off preventive mechanism  9  is installed freely in advance or retreat in the perpendicular form to the headrest stay  3   b . The headrest stay  3   b  is inserted through the falling-off preventive mechanism  9 . The falling-off preventive mechanism  9  comprises the lock plate  10  and the spring  12  (not shown). The lock plate  10  comprises a button  10   b  to unlock an abutment (not shown) in contact with the headrest stay  3   b . The falling-off preventive notch  5  is formed by a horizontal surface  5   a , and an inclined surface  5   b  to increase the diameter slowly upward from the deepest portion of the horizontal surface  5   a . When the lock plate  10  reaches the position of the falling-off preventive notch  5 , the abutment (not shown) of the lock plate  10  engages the horizontal surface  5   a  of the falling-off preventive notch  5 . This allows to lock the movement to the upper portion of the headrest stay  3   b . When the locked movement is unlocked, by pushing the outside exposed button  10   b  of a housing  13  the abutment (not shown) disengages the horizontal surface  5   a  of the falling-off preventive notch  5 , and the locked headrest stay  3   b  is unlocked. 
     However, in the headrest busing  3   b  as mentioned above when the falling-off preventive mechanism  9  unlocks the locked headrest stay  3   b , the button  10   b  for unlocking the locked headrest stay  3   b  is exposed to the outside of the housing to accommodate the falling-off preventive mechanism  9 . Accordingly, even supposing that the occupant merely touches, if the button  10   b  is pushed, the locked movement to the upper portion of the headrest  2  may be sufficiently unlocked. Even though the lock may be unlocked with such a mere touch, the headrest may be disengaged when the occupant does not intend, and a dangerous situation also may occur. 
     The present invention is to provide the headrest bushing having the headrest  2  not dislocated from the seatback  1  and the vehicle seat therewith, if the invention is performed, not by an intentional action forcing the headrest  2  to be disengaged from the seatback  1 . 
     SUMMARY OF THE INVENTION 
     A headrest bushing into which a headrest stay are inserted for supporting a head rest according to one embodiment of the present invention comprising: 
     a lock mechanism for preventing disengagement of said headrest stay from said headrest bush; and 
     an unlock mechanism having a button for unlocking said lock mechanism and a housing for accommodating said lock mechanism, which has an opening for manipulating said button; and 
     wherein a surface of said button is placed inwardly from said opening of said housing. 
     A vehicle seat according to one embodiment of the present invention comprising a seatback and a headrest, said headrest comprising: 
     a headrest main body; 
     a first and second headrest stays for supporting said headrest main body; and 
     a first and second headrest bushes into which said first and second headrest stays are inserted respectively; said first headrest bush comprising:
         a first lock mechanism for preventing disengagement of said first headrest stay from said first headrest bush; and   a first unlock mechanism having a first button for unlocking said first lock mechanism and a housing for accommodating said first lock mechanism, which has an opening for manipulating said button; and   wherein a surface of said first button is placed inwardly from said opening of said housing.       

     A vehicle seat according to one embodiment of the present invention comprising a seatback and a headrest, said headrest comprising: 
     a headrest main body; 
     a first and second headrest stays for supporting said headrest main body; and 
     a first and second headrest bushes into which said first and second headrest stays are inserted respectively; 
     said first headrest bush comprising:
         a first lock mechanism for preventing disengagement of said first headrest stay from said first headrest bush; and   a first unlock mechanism having a first button for unlocking said first lock mechanism and a first housing for accommodating said first lock mechanism, which has a first opening for manipulating said first button;   wherein a surface of said first button is placed inwardly from said first opening of said first housing; and       

     said second headrest bushing comprising:
         a second lock mechanism for preventing said second stay from moving to the lower portion; and   a second unlock mechanism having a second button for unlocking said second lock mechanism and a second housing for accommodating said second lock mechanism, which has a second opening for manipulating said second button;       

     wherein a surface of said first button is placed inwardly from said first opening of said first housing; and 
     wherein a surface of said second button protrudes outside from said second housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing headrest bushings and one preferred embodiment of a vehicle seat of the present invention. 
         FIG. 2  is a main exploded perspective view showing the headrest bushings and a preferred embodiment of a vehicle seat according to the present invention. 
         FIG. 3  is a partial cutaway perspective view of a height adjusting mechanism showing a preferred embodiment according to the invention. 
         FIG. 4  is a partial cutaway perspective view of a disengagement preventive mechanism showing a preferred embodiment according to the invention. 
         FIG. 5  is a perspective view showing a detailed description of a preferred embodiment of the present invention which is in a state covered by a housing. 
         FIG. 6A  and  FIG. 6B  are sectional views which cut away the disengagement preventive mechanism of  FIG. 5  with line a-a′, respectively. 
         FIG. 7  is an exploded perspective view showing a preferred embodiment according to the invention. 
         FIG. 8  is a perspective view showing other detailed description of a preferred embodiment of the present invention which is in a state covered to a housing. 
         FIG. 9A ,  FIG. 9B  and  FIG. 9C  are sectional views which cut away the disengagement preventive mechanism of  FIG. 8  with b-b′, respectively. 
         FIG. 10  is a main exploded perspective view showing a vehicle seat comprising only a conventional height adjusting mechanism. 
         FIG. 11  is a main exploded perspective view showing the vehicle seat comprising a conventional falling-off preventive mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 1 
     Referring now to the drawings from  FIG. 1  to  FIG. 9 , one embodiment according to the present invention is described. The vehicle seat and identical components which are shown in  FIGS. 10 and 11  have the same numeral, a description is made. 
     Referring now to  FIG. 1 ,  FIG. 1  depicts a perspective view showing a preferred embodiment according to the invention. The numeral  1  of  FIG. 1  depicts a seatback,  2  of  FIG. 1  may be described as a headrest main body, the main body (hereinafter merely referred as to a “headrest”) forming a headrest body according to this embodiment of the invention, and  3   a  and  3   b  are designated as headrest stays. 
     Referring now to  FIG. 2 ,  FIG. 2  depicts a main exploded perspective view showing a preferred embodiment according to the invention. The numeral A of  FIG. 2  depicts a headrest portion comprising of a headrest  2 , headrest stays  3   a  and  3   b , and headrest bushings  6   a  and  6   b . The numeral B of  FIG. 2  is designated as a seatback. In the headrest  2 , the headrest stays  3   a  and  3   b  are supported to be inserted into the headrest bushings  6   a  and  6   b  in the form of tube anchored at the upper end position of a seatback  1 . There are provided with two of the headrest stays  3   a  and  3   b  installed in right and left. The headrest bushings  6   a  and  6   b  are fixedly mounted to the seatback frame  11  of the seatback  1  as shown in  FIG. 7 . More particularly, a tube-shaped mounting member  14  is connected to a seatback frame  11 , and is fixed to be inserted into the mounting member  14 . In the headrest bushings  6   a  and  6   b  one of the headrest bushings  6   a  has the height adjusting mechanism  7 , and a disengagement preventive mechanism  21  is arranged to the other. By the height adjusting mechanism  7  the headrest  2  (the headrest stay  3   a ) are halted (held) at a given height, and by the disengagement preventive mechanism  21  the headrest  2  (the headrest stay  3   b ) is composed for prevention of falling-off. What is a “disengagement preventive mechanism” herein called is the mechanism comprising: a locking mechanism for preventing disengagement of the headrest stay  3   b  inserted into the headrest bushing  6   b , a button for unlocking the locking mechanism, and an unlocking mechanism having a housing to accommodate the button. 
     The height adjusting mechanism  7  is arranged to the upper end of the seatback  1  of the headrest bushing  6   a .  FIG. 3  depicts a partial cutaway perspective view of the height adjusting mechanism  7 . The height adjusting mechanism  7  in  FIG. 3  is described in detail. In the height adjusting mechanism  7 , the lock plate  8  as a locking member to the headrest bushing  6   a  is attached in advance or retreat in a perpendicular direction to the headrest stay  3   a . The headrest stay  3   a  passes through the lock plate  8  to be inserted into the headrest bushing  6   a . The headrest stay  3   a  has the space that the lock plate  8  can move (translate back and forth) therebetween, a portion  8   a  of the lock plate  8  is abutted on the perforated headrest stay  3   a  by a resilience of the spring  12 . The lock plate  8  and the spring  12  are accommodated in the housing  13 , and only the button  8   b  of the lock plate  8  is protruded. It may be allowed to unlock the lock plate  8  by the mere pushing of the button  8   b . In the headrest stay  3   a  a plurality of the height adjusting notches  4  are arranged in a predetermined space. The height adjusting notches  4  are formed of the horizontal surface  4   a  toward the center of the headrest stay  3   a , and an inclined surface  4   b  to increase the diameter of slowly downward from the deepest part of the horizontal surface  4   a.    
     Therefore, by pulling (falling-off) the headrest  2  upward an abutment  8   a  in touch with the headrest stay  3   a  of the lock plate  8  is disengaged from the height adjusting notches  4  against an elastic force (a spring force) of the spring  12  by action of the incline surface  4   b  of the height adjusting notches  4 , so that the headrest stay  3   a  is movable upward, and the headrest  2  adjustable in its height. In the movement to the lower portion, the horizontal surface  4   a  of the height adjusting notches  4  engage the lock plate  8  to be locked and halted in place. On the contrary, the lock plate  8  is not disengaged from the height adjusting notches  4  in engagement with a horizontal surface  4   a  of the height adjusting notches  4  when the headrest  2  is pushed downward. Thus, when the button  8   b  of the lock plate  8  is pushed, then the lock is unlocked, the movement may be available to the lower portion of the headrest stay  3   a , so that the headrest  2  can be regulated downward. 
     Referring now to  FIG. 4 ,  FIG. 4  depicts partial cutaway perspective view of the disengagement preventive mechanism. The disengagement preventive mechanism  21  is arranged on an upper end position of the seatback  1  of the headrest bushing  6   b . The disengagement preventive mechanism  21  has a generally same construction as the height adjusting mechanism  7 , but there is a difference in which the falling-off preventive notch  5  is arranged in the lower portion of the inserted headrest stay  3   b . That is, in the disengagement preventive mechanism  21 , as shown in  FIG. 4 , the lock plate  22  as a locking member to the headrest bushing  6   b  is attached in advance or retreat in a perpendicular direction to the headrest stay  3   b . The headrest stay  3   b  passes through the lock plate  22  to be inserted into the headrest bushing  6   b . A portion that the headrest stay  3   b  passes through the lock plate  22  has a space for allowing the lock plate  22  to move front and back between the headrest stay  3   b  and the lock plate  22 . A portion  22   a  of the lock plate  22  is abutted on the perforated headrest stay  3   b  by a resilience of the spring  12 . The lock plate  22  and the spring  12  are accommodated in the housing  20 . By pushing the button  22   b  formed a part of the lock plate  22 , it may be allowed to unlock the lock plate  22 . Subsequently, the falling-off preventive notch  5  is arranged to the lower portion of the headrest stay  3   b . The falling-off preventive notch  5  is formed of the horizontal surface  5   a  toward the center of the headrest stay  3   b , and an inclined surface  5   b  to increase the diameter slowly upward from the deepest part of the horizontal surface  5   a.    
     Accordingly, the headrest stay  3   b  is free to move up and down. However, when the falling-off preventive notch  5  moves up and down so as to reach the position of the lock plate  22 , the lock plate  22  engages the falling-off preventive notch  5 , thereby locking the horizontal surface  5   a  and preventing the up and down movement, so that the headrest stay  3   b  is not able to move up and down anymore, thereby allowing the headrest  2  to be prevented from falling-off. When the headrest stay  3   b  is further moved up and down, and is pulled out, the lock plate  22  can be unlocked by the pushing of the button  22   b . More particularly, when the locked movement to the lower portion of the headrest stay  3   b  is unlocked by the pushing of the button  22   b , without being enough by a weak pressing force of the extent such as a mere touch, the pressing force is required such that the engagement between the lock plate  22  and the horizontal surface  5   a  of the falling-off preventive notch  5  is disengaged. 
     In the portion that the headrest stay  3   a  and  3   b  in  FIG. 3  and  FIG. 4 , by providing with a space that the lock plates  8  and  22  can move (translate back and forth) between the headrest stays  3   a  and  3   b  and the lock plates  8  and  22 , the lock plates  8  and  22  can be pressed and moved to unlock the engagement (lock) with the height adjusting notches  4  and the falling-off preventive notch  5 . Accordingly, the space is required when unlocking from the height adjusting notches  4  of the lock plates  8  and  22 , and the falling-off preventive notch  5 . 
     As set forth in  FIGS. 3 and 4 , in the height adjusting mechanism  7 , by pulling the headrest  2  upward the abutment  8   a  in touch with the headrest stay  3   a  of the lock plate  8  is disengaged from the height adjusting notch  4  against an elastic force of the spring  12  by action of the incline surface  4   b  of the height adjusting notch  4 , so that the headrest stay  3   a  is movable upward, and the headrest  2  adjustable in its height. On the contrary, the lock plate  8  is not disengaged from the height adjusting notches  4  in engagement with the horizontal surface  4   a  of the height adjusting notches  4  when the headrest  2  is pushed downward. Consequently, when the button  8   b  of the lock plate  8  is pushed, then the lock is unlocked against the spring  12 , the movement may be available to the lower portion of the headrest stay  3   a , so that the headrest  2  can be regulated downward. As such, the height adjusting mechanism  7  of this embodiment of the invention can adjust its height by pulling the headrest  2  upward; the lock plate  8  may be unlocked by pushing the button  8   b  to be thereby depressed downward. The control button  8   b  is located on the upper end of the seatback, which can be easily manipulated. 
     Similarly, as set forth in  FIGS. 3 and 4 , even though the disengagement preventive mechanism  21  moves to the upper potion of the headrest  2 , when the falling-off preventive notch  5  of the headrest stay  3   b  reaches the position of the lock plate  22 , the lock plate  22  engages the falling-off preventive notch  5 , thereby locking the horizontal surface  5   a  and preventing from falling off. 
     Referring now to  FIG. 5  and  FIG. 6 , a detailed description is made of the falling-off preventive mechanism.  FIG. 5  shows a perspective view of the housing  20  that accommodates the lock plate  22  and the spring  12  of the falling-off preventive mechanism  21 . This allows the lock plate  22  and the spring  12  of the falling-off preventive mechanism  21  to be in the state covered by the housing  20 , there is an opening on the surface of the housing  20  in internal contact with the button  22   b  of the lock plate  22 . Hereinafter the portion having an opening surface of the housing  20  may be referred as to an “opening”. This allows the button  22   b  to be manipulated. 
       FIG. 6A  and  FIG. 6B  is a sectional view which cuts away the housing  20  accommodating the lock plate  22  and the spring  12  of the falling-off preventive mechanism  21  with line a-a′. On the left upper end of the headrest bushing  6   b  the abutment  22   a , which is a portion of the lock plate  22  in contact with the headrest stay  3   b , is located, and on the right upper end of the headrest bushing  6   b  the button  22   b  is located for unlocking the horizontal surface  5   a  of the falling-off preventive notch  5  arranged to the lock plate  22  and the headrest stay  3   b .  FIG. 6A  and  FIG. 6B  can be seen in the state having the lock plate  22  separated in right and left sides due to the fact that  FIG. 5  is taken along line a-a′, but as mentioned above the abutment  22   a  and the operating button  22   b  are integrated into one of the lock plate  22 . Similarly, the spring  12  for resisting the lock plate  22  from the headrest stay  3   b  is arranged behind the lock plate  22 . 
       FIG. 6A  is a view showing the state that the surface of the button  22   b  of the lock plate  22  corresponds to the opening of the housing  20 .  FIG. 6B  is a view showing the state that the surface of the button  22   b  of the lock plate  22  was placed inwardly in a pressing direction from the opening of the housing  20 . By such an arrangement the button  22   b  would be not pushed and unlocked without implementing an occupant intentional work. 
     When the lock plate  22  of the headrest stay  3   b  is unlocked to further pull out the headrest  2 , the headrest  2  can be pulled out by the pushing of the button  22   b  of the lock plate  22  for unlocking. More particularly, when the headrest  2  is pulled out, the lock plate  22  is unlocked by the pushing of the button  22   b  for raising the headrest  2  to the most subjacent notch  4  among a plurality of the height adjusting notches  4  of the headrest stay  3   a  in this embodiment of the present invention. This work can be manipulated with one hand. Next, the most subjacent notch  4  is unlocked, and the headrest  2  is raised to the falling-off preventive notch  5  of the headrest stay  3   b  in this embodiment of the invention. This work can also be manipulated with one hand. Furthermore, while raising the headrest  2  with one hand, the headrest  2  is unlocked by the pushing of the button  22   b  formed a portion of the lock plate  22  with another hand, and is removed. The numbers of the notches are not limited to the above-mentioned numbers. 
     According to this embodiment of the present invention, the falling-off preventive mechanism  21  is arranged to the upper end of the seatback  1  as shown in  FIG. 1  and  FIG. 2 , unlikely a conventional vehicle seat that the falling-off preventive mechanism  9  is arranged in the inside of the seatback  1 , and it allows to work easily when the headrest is removed from the seatback upon cleaning up or maintenance. 
     In this embodiment, the position that the falling-off preventive notch  5  of the headrest stay  3   b  of the headrest  2  reaches and locks the position of the lock plate  22  corresponds to the position that the lock plate  8  is locked to the most subjacent height adjusting notch  4  of the headrest stay  3   a  in the height adjusting mechanism  7 , when the falling-off preventive mechanism  21  locks the headrest  2  to be pulled out upward, even if an external force is removed, the headrest  2  is preferable to hold its position. 
     Referring to the vehicle seat according to the invention, the locking mechanism and the unlocking mechanism for preventing disengagement from the seatback of the headrest are covered by the housing with the opening, the button should be pushed at the strength of the extent allowing to be unlocked. In this is manner, it would not allow to unlock without implementing a human intentional work. 
     Therefore, “A headrest is uncapable of being removed unless the headrest is made into an internal action that is clearly different from an action required for adjustment.” is defined and satisfied in U. S. Federal Motor Vehicle Safety Standards (hereinafter referred as to “FMVSS”) 202aS4 clause 5, and it can be thought that safety is secured enough. 
     Embodiment 2 
     Referring now to  FIG. 8 ,  FIG. 8  depicts a vehicle seat of the other embodiment according to the present invention. The numeral  30  of  FIG. 8  depicts the housing covering the disengagement preventive mechanism  21 , and  32   b  depicts a button for unlocking the lock plate  32  of the disengagement mechanism  21 . The housing  30  has an opening for opening the surface opposed to the pressing surface of the button  32   b  of the lock plate  32 . The surface of the button  32   b  is arranged in a pressing direction from the opening of the housing  30 , in other words, at the inside of the housing  30  (including the housing that corresponds to the opening of the housing  30 ). As this embodiment of the present invention only differs in the housing of the disengagement preventive mechanism and the shape (width dimension) of the locking member, and other component uses the same member as same as the embodiment 1 is used, the description of which is omitted. 
     Referring now to  FIG. 9 ,  FIG. 9A  depicts a sectional view of the disengagement preventive mechanism for use in a conventional vehicle seat as shown in  FIG. 10 .  FIG. 9B  depicts a sectional view of the disengagement preventive mechanism of the embodiment 1 according to the present invention described above.  FIG. 9C  depicts a sectional view of the disengagement preventive mechanism which cuts away with line b-b′ of  FIG. 8  of this embodiment 3 according to the present invention. 
     In this embodiment according to the invention, the width dimension of the housing  30  is identical as the housing mounted to a conventional disengagement preventive mechanism, but the lock plate  32  is shorter in its width dimension than a conventional locking member  12 . This allows the lock plate  32  to be covered by the housing  30 , so that the button  32   b  is accommodated in the housing  30  without being protruded. As the button  32   b  is arranged to the inside of the housing  30 , if free of intentionally pushing the button  32   b , the lock plate  32   b  may not be unlocked unless the button  32   b  is intentionally pushed. 
     The embodiment described herein is not intended to limit the present invention, various modifications can be made without departing from the scope and sprit of the present invention. 
     Embodiment 3 
     The vehicle seat according to the present invention can be applied to not only a motor vehicle but also other vehicle such as a railway and an airplane. Additionally, the headrest bushing according to the invention can be applied to not only a vehicle seat but also other vehicle seat.
A headrest bushing into which a headrest stay are inserted for supporting a head rest includes a lock mechanism for preventing disengagement of the headrest stay from the headrest bush; and an unlock mechanism having a button for unlocking the lock mechanism and a housing for accommodating which has an opening for manipulating the button, and wherein a surface of the button is placed inwardly from the opening of the housing. The surface of the button is placed correspondently to the opening of the housing or inwardly in a pressing direction from the opening of the housing. The headrest stay has a notch consisting of a horizontal surface and an inclined surface to increase its diameter of slowly downward from the deepest part of the horizontal surface, and wherein the horizontal surface of the notch engages with the lock mechanism.