BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a baseball bat used in baseball game. 
         [0003]    2. Description of the Related Art 
         [0004]    A baseball bat must withstand violent shocks at the time of batting, and the kind of wood capable of being used as a bat for regulation baseball is limited to serrata, maple, white ash, and the like. The growing district of these kinds of wood is restricted, and the raising thereof requires many years. In recent years, therefore, wood resources have run out, and the price of wooden bat tends to go up. As a substitute for the wooden bat, a metal bat has been known. However, because of its light weight and high coefficient of rebound, the metal bat has a problem in that the metal bat may offer a one-sided benefit for the offensive team, and thus smooth development of a game cannot be hoped for. Therefore, in professional baseball, the use of metal bat is not admitted. In amateur baseball as well, some sort of restriction is sometimes imposed on the use thereof. 
         [0005]    From the above-described background, as a substitute for the conventional wooden bat, a laminated bat manufactured by bonding wood pieces or bamboo pieces has come into wide use. The laminated bat has an advantage that valuable forest resources can be saved, and moreover the price can be kept low. Therefore, although the use in professional baseball is not admitted, in amateur baseball, the use thereof is partially admitted in regular match. Although various kinds of laminated bats are available, the most popular one is a bamboo bat in which the whole of the bat is formed by bamboo pieces. The bamboo bat is made of bamboo pieces obtained by cutting a felled moso bamboo tree into slender and thin rectangular strips, which are bonded to each other with an adhesive to make a base material having a square cross section, and then the base material is finished into a predetermined shape by using a lathe. As a laminated bat other than the bamboo bat, a product called a “lami bat” has been used widely, which is manufactured by forming only the central part thereof by bamboo pieces and by affixing a plate material such as maple to around the central part. In construction, this lami bat is configured so that the grip part thereof is completely made of bamboo, while the outer periphery of the hitting part thereof is covered with wood equivalent to that of the wooden bat. 
         [0006]    The technological development concerning a bat has a long history, and a large number of patents and the like have so far been applied for. Among these, applications relating to the present application are described below. Patent Document 1 is characterized by penetratingly providing rattan wood in the center of bat to improve the strength of the bat. Patent Document 2 is characterized by inserting an iron rod in the center of bat to prevent breakage and to improve the strength. Patent Document 3 relates to the construction of bamboo bat, and discloses a bamboo piece arranging method and the like. Patent Document 4 is characterized in that in order to improve the rebounding force of bamboo bat, a hard material is used in the central part, and a bamboo material is bonded to both sides thereof to form a hitting part. 
       Patent Documents 
       [0007]    Patent Document 1: Japanese Utility Model No. M4262 
         [0008]    Patent Document 2: Japanese Utility Model No. M63303 
         [0009]    Patent Document 3: Japanese Utility Model No. M379399 
         [0010]    Patent Document 4: Japanese Utility Model Publication No. 35-016213 
         [0011]    Bamboo, which is a material for a bamboo bat, grows fast and has no problem in terms of resource conservation, and an adhesive for bamboo has been improved so that the bonded portion of bamboo is rarely peeled off by shocks and elapse of time. As well known, however, bamboo is deformed easily because of its low rigidity against a bending load. This property remains still in a bamboo bat, which is a bamboo product. Therefore, the bamboo bat has a problem in that at the time of batting, the whole thereof is deformed into an arch shape, resulting in a shorter flying distance, so that the bamboo bat is unsuitable for the use in regular match. The aforementioned lami bat also has the same problem because it is formed by bamboo except for the outer periphery. Therefore, in amateur baseball as well, it is difficult to wholly introduce the bamboo bat and the lami bat, and a wooden bat must be provided separately. However, the wooden bat is expensive as described above, and moreover the strength thereof varies from product to product, so that the wooden bat may be broken by the use of only several times. 
         [0012]    The specific gravity of bamboo is about 0.68, while that of serrata, maple, and the like is about 0.73. Although the specific gravity differs depending on the kind and growing district, the degree of drying after felling, and the like, nevertheless bamboo is always lighter in weight. Therefore, as a product, if the bat shape is identical, the bamboo bat is lighter than the wooden bat. Also, if the bat weight is identical, the wooden bat is slenderer than the bamboo bat. Therefore, it is difficult to put the shapes and weights of bamboo bat and wooden bat in agreement with each other, so that a difference arises in their feel in use. Incidentally, the weight of wooden bat for regulation baseball is most generally in the range of 880 to 910 grams. 
         [0013]    Both of the wooden bat and the laminated bat have a limited strength because natural resources are used, so that they may break at the time of batting. In particular, in the grip part, bending moment acting between a hand gripping the bat and a batted ball becomes at the maximum, and moreover the cross-sectional area of this part is decreased, so that an excessive stress is liable to develop and thereby the grip part is broken easily. This problem can be solved by providing some core material as described in Patent Document 1, for example. However, the core material having a circular cross section as described in Patent Document 1 has a problem in that it is difficult to apply a pressure to the outer peripheral surface thereof, so that the bonding strength becomes slightly insufficient and thereby the bond is easily broken by a shock. In the case where a metal or a resin other than wood and bamboo is used as the core material as described in Patent Document 2, it may be difficult to use the bat in regular match according to the “authorized baseball rule” specified by the game organization, so that the use of this bat is limited to knock batting and the like. 
         [0014]    Constructed of a hard material in the whole of the grip part, the bat described in Patent Document 4 has a problem in that since the weight thereof increases as compared with a general wooden bat, the load increases when it is swung, and also the shock at the time of batting is transmitted to the hand without being buffered, resulting in its uncomfortable feel in use. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention has been made in view of the above circumstances, and accordingly an object thereof is to provide a baseball bat that has performance similar to that of the conventional wooden bat and moreover is excellent in terms of cost and resource conservation. 
         [0016]    To achieve the above object, the present invention provides a baseball bat including a reinforcing core arranged in the center of the cross section thereof; and an outer peripheral material that surrounds the reinforcing core and is formed by collecting laminated bodies of bamboo pieces, wherein the reinforcing core and individual bamboo pieces are integrated by bonding; and the reinforcing core has a rectangular cross section, and is provided penetratingly from a distal end to a proximal end of bat so that both of the reinforcing core and the bamboo pieces exist in all of the transverse cross sections from the distal end to the proximal end of bat. 
         [0017]    The baseball bat in accordance with the present invention is based on the conventional bamboo bat, but is characterized by incorporating the reinforcing core therein. The cross section of the reinforcing core is limited to a rectangular shape, and moreover the reinforcing core is of a one-rod shape that is continuous from the distal end on the hitting side to the proximal end on the grip side, so that both of the reinforcing core and the bamboo pieces always exist in all of the transverse cross section from the distal end to the proximal end of bat, and there is no transverse cross section in which only the reinforcing core or only the bamboo pieces exist. The transverse cross section is a circular cross section intersecting at right angles with the lengthwise direction of bat. However, the center of the reinforcing core need not be in agreement with the bat center, and the bat center has only to lie at any position in the cross section of reinforcing core. Also, since the reinforcing core is provided to enhance the strength of bat, the material thereof is required to have a flexural rigidity higher than that of bamboo and a specific gravity larger than that of bamboo. Considering the use in regular match, as the material of reinforcing core, a natural material such as wood or bamboo is used. However, the material is not limited to a solid one, and any of various bonded materials can also be used. 
         [0018]    The bamboo piece laminated around the reinforcing core is the same as that used in the conventional bamboo bat, and is manufactured by tearing a felled bamboo tree in the lengthwise direction and by cutting the torn piece into a predetermined length, so that the cross section thereof is of a slender rectangular shape, and the bamboo piece has an appearance having a shape of a rectangular paper strip. An adhesive is applied to the broad surface of bamboo piece, the broad surfaces are lapped on each other while being brought into contact with each other, and then a pressure and heat are applied thereto, by which a laminated body of bamboo pieces is manufactured. Thereafter, an adhesive is applied to one surface of the laminated body, the reinforcing core is brought into contact with this surface, and a pressure and heat are applied again, by which the reinforcing core is integrated with the bamboo pieces. By repeating this process, a square rod shaped base material in which the bamboo pieces are laminated around the reinforcing core without gap is completed. By turning the base material into a predetermined shape by using a lathe or the like, a product is completed. 
         [0019]    By limiting the cross section of reinforcing core to a rectangular shape as described above, a pressure can be applied to the contact surface between the reinforcing core and the bamboo pieces when the base material is manufactured. Therefore, ideal surface contact is realized without distortion of the bamboo pieces and the like. In addition, since the adhesive permeates evenly throughout the contact surface, the reinforcing core is substantially integrated with the bamboo pieces. Therefore, the load applied to the outer periphery of bat is surely transmitted to the reinforcing core, and the formation of cracks caused by poor bonding can be prevented. Also, since the reinforcing core is penetratingly provided continuously, the rigidity is improved in all regions, so that the deformation of bat at the time of batting can be restrained. Further, due to the difference in specific gravity between the reinforcing core and bamboo, the weight is increased as compared with a pure bamboo bat, and the feel in use is also improved. The baseball bat in accordance with the present invention is basically an improvement of bamboo bat, and therefore has no portion in the whole of transverse cross section which is composed only of the reinforcing core, so that the rigidity is not increased excessively by the reinforcing core. 
         [0020]    In the present invention, the material of the reinforcing core is preferably hickory. Hickory, which is a broad-leaved tree in the walnut family mainly growing in the eastern district of North America, has a higher hardness and shock absorbing ability by nature as a wood material. The specific gravity thereof in a dried state is about 0.85. By using hickory as the reinforcing core as described above, the rigidity of bat is increased while the cross-sectional area of reinforcing core is restrained. Therefore, the flexibility peculiar to the bamboo bat is improved, and also the weight is increased due to the difference in specific gravity, so that the feel in use similar to that of the wooden bat can be obtained by the optimization of cross-sectional area thereof. The resources of hickory is abundant, and therefore the availability thereof offers no problem. 
         [0021]    In the present invention, the material of the reinforcing core is preferably reinforced wood. The reinforced wood, which is artificial wood manufactured by impregnating a wooden plate with phenolic resin or the like, has improved strength and hardness as compared with natural wood, and therefore has been used as a building material and the like. The specific gravity of reinforced wood varies depending on the kind thereof, and sometimes exceeds 1. Therefore, the reinforced wood can provide rigidity and weight approximate to those of the wooden bat while the cross-sectional area of reinforcing core is restrained. As the reinforcing core, in addition to the aforementioned hickory and reinforced wood, Japanese white birch or evergreen oak can also be chosen. 
         [0022]    In the present invention, each side of the cross section of the reinforcing core preferably has a length not shorter than 15 mm and not longer than 20 mm. By restricting the size in this manner, the cross-sectional shape of reinforcing core is made approximately square, so that the difference in flexural rigidity between directions is small. In the case where the reinforcing core has a square cross section of 15 mm×15 mm, if the specific gravity of reinforcing core is 0.85 and the overall length of bat is 840 mm, the weight of bat increases by about 30 g as compared with a pure bamboo bat. 
         [0023]    In the present invention, one side of the cross section of the reinforcing core preferably has a length not shorter than 30 mm and not longer than 40 mm, and the other side thereof preferably has a length not shorter than 5 mm and not longer than 12 mm. By restricting the size in this manner, the cross-sectional shape of reinforcing core is made rectangular, and thereby the geometric moment of inertia in the long side direction is increased. Therefore, even at the time of batting, high rigidity can be obtained. Also, when a load is applied in the short side direction, high rigidity can be obtained as compared with the bamboo bat. In the present invention, since one side of reinforcing core is relatively large, being not shorter than 30 mm, in the grip part in which the cross-sectional area is decreased, the side surfaces of the reinforcing core are exposed to the outside. In this exposed part only, the cross section of reinforcing core does not have a rectangular shape, but has an elongated circular shape, and bamboo pieces are laminated on both sides thereof. 
         [0024]    In the present invention, as described above, the reinforcing core is incorporated in the center of the baseball bat, and the bamboo pieces are laminated at the outer periphery of the reinforcing core. Therefore, the flexibility peculiar to the bamboo bat is improved, and rigidity similar to that of the wooden bat is obtained. In addition, the cross-sectional area is optimized by using a material having specific gravity larger than that of bamboo as the reinforcing core. Thereby, the weight can be adjusted so as to be equivalent to that of the wooden bat, so that the feel in use at the time of batting and the behavior of a batted ball are improved greatly as compared with the case where the conventional bamboo bat or lami bat is used. Moreover, in the present invention, since both of the reinforcing core and the bamboo pieces exist in all regions of bat, the weight is not larger than that of the wooden bat, and the rigidity does not increase excessively. Therefore, the shock at the time of batting is buffered properly. 
         [0025]    Also, the reinforcing core need not use a scarce material, so that a low-cost and high-performance bat using readily available materials can be supplied to the market. Further, since the reinforcing core of the present invention has a rectangular cross section, and the bonding work can be performed while a pressure is applied to the adjacent contact surfaces, the reinforcing core and the bamboo pieces are integrated firmly with each other, so that breakage originating from the interior can be prevented. In addition, since the reinforcing core is provided continuously so as to penetrate both ends of the bat, the strength of the grip part in which the cross-sectional area is decreased is improved, and the durability is also increased. Since both of the reinforcing core and the bamboo pieces exist in the grip part as well, the buffering of shock can be anticipated in addition to the improvement in strength. Also, the bat in accordance with the present invention can be manufactured by making use of the manufacturing process for the conventional bamboo bat, so that the product price can be kept low in this respect as well. 
         [0026]    By using hickory as a material for the reinforcing core as described above, due to high flexural rigidity and large specific gravity, which are features of hickory, the performance equivalent to that of the wooden bat can be obtained while the cross-sectional area of reinforcing core is kept small. Therefore, the manufacturing process is almost the same as that for the conventional bamboo bat. Moreover, hickory has no problem with availability, and therefore can keep the product price low. Further, hickory is a kind of natural wood, so that when a bat is manufactured by incorporating hickory, the bat can be handled as one kind of laminated bats. 
         [0027]    By using reinforced wood as a material for the reinforcing core as described above, the rigidity and specific gravity of reinforcing core can be adjusted artificially, so that the rigidity and weight of bat can be changed as desired, and therefore a bat that fits its purpose can be provided. Moreover, since reinforced wood has been used in various fields including buildings, it has no problem in terms of resource availability and cost, and can keep the product price low. Further, if being approved by the game organization, the bat using hickory can also be used in regular match. 
         [0028]    By limiting the cross section of reinforcing core to a square shape having the predetermined size or a rectangular shape approximate to the square shape as described above, rigidity and weight similar to those of the wooden bat is obtained, and also the rigidity of bat is equal in all directions. Therefore, at time of batting, attention need not be paid to how to grip the bat. 
         [0029]    By limiting the cross section of reinforcing core to a rectangular shape having the predetermined size as described above, in the case where the long side direction of reinforcing core is put in agreement with the hitting direction, high rigidity is obtained and the bat performance is improved. In addition, the cross-sectional area of reinforcing core does not become excessively large, and an increase in weight than necessary can be restrained. Moreover, since the bamboo pieces exist continuously even in the grip part, some degree of shock absorption can be anticipated. Also, in this mode, since the side surfaces of the reinforcing core are exposed to the outside in the grip part, the use of two kinds of materials, the reinforcing core and the bamboo, can be confirmed visually. Therefore, the bat using the reinforcing core having this shape can be regarded as one kind of laminated bats equivalent to a lami bat and the like. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is views showing a construction example of a baseball bat in accordance with the present invention, in which a reinforcing core has a square cross section,  FIG. 1(A)  being a front view and a left side view,  FIG. 1(B)  being a sectional view taken along the line B-B of  FIG. 1(A) ,  FIG. 1(C)  being a sectional view taken along the line C-C of  FIG. 1(A) , and  FIG. 1(D)  being a sectional view taken along the line D-D of  FIG. 1(A) ; 
           [0031]      FIG. 2  is sectional views of a base material of the baseball bat shown in  FIG. 1 ,  FIG. 2(A)  being a sectional view showing a state in which constituent elements are separated, and  FIG. 2(B)  being a sectional view showing a state in which all of the constituent elements are integrated; 
           [0032]      FIG. 3  is views showing a construction example of a baseball bat in accordance with the present invention, in which a reinforcing core has a rectangular cross section,  FIG. 3(A)  being a front view and a left side view,  FIG. 3(B)  being a sectional view taken along the line B-B of  FIG. 3(A) ,  FIG. 3(C)  being a sectional view taken along the line C-C of  FIG. 3(A) , and  FIG. 3(D)  being a sectional view taken along the line D-D of  FIG. 3(A) ; 
           [0033]      FIG. 4  is sectional views of a base material of the baseball bat shown in  FIG. 3 ,  FIG. 4(A)  being a sectional view showing a state in which constituent elements are separated, and  FIG. 4(B)  being a sectional view showing a state in which all of the constituent elements are integrated; 
           [0034]      FIG. 5  is a front view of a baseball bat in accordance with the present invention, showing an outline at the time when the flexural rigidity thereof is measured; 
           [0035]      FIG. 6  is a graph showing the measurement results of the flexural rigidity of a baseball bat used conventionally; 
           [0036]      FIG. 7  is a graph showing the measurement results of the flexural rigidity of a baseball bat in accordance with the present invention, in this baseball bat, hickory being used as a material for a reinforcing core, and the reinforcing core having a square cross section having one side of 16 mm; 
           [0037]      FIG. 8  is a graph showing the measurement results of the flexural rigidity of a baseball bat in accordance with the present invention, in this baseball bat, hickory being used as a material for a reinforcing core, the reinforcing core having a rectangular cross section having a long side of 36 mm and a short side of 10 mm, and the long side being in agreement with the load direction; 
           [0038]      FIG. 9  is a graph showing the measurement results of the flexural rigidity of a baseball bat in accordance with the present invention, in this baseball bat, hickory being used as a material for a reinforcing core, the reinforcing core having a rectangular cross section having a long side of 36 mm and a short side of 10 mm, and the short side being in agreement with the load direction; 
           [0039]      FIG. 10  is a front view of a baseball bat in accordance with the present invention, showing an outline of experiment at the time when the flexural rigidity thereof is measured under a condition close to the actual batting; 
           [0040]      FIG. 11  is a graph showing the measurement result of the flexural rigidity of a baseball bat having a reinforcing core of a square cross section, the measurement result being obtained by the experimental method shown in  FIG. 10 , in this baseball bat, hickory having a square cross section having one side of 16 mm being used as a material for the reinforcing core; 
           [0041]      FIG. 12  is a graph showing the measurement results of the flexural rigidity of a baseball bat having a reinforcing core of a rectangular cross section, the measurement results being obtained by the experimental method shown in  FIG. 10 , in this baseball bat, hickory having a rectangular cross section having a long side of 36 mm and a short side of  10  mm being used as a material for the reinforcing core; 
           [0042]      FIG. 13  is front views of a baseball bat in accordance with the present invention, showing an outline of experiment at the time when a hard ball is caused to collide with the baseball bat or the like and the speed of the hard ball immediately after the collision is measured,  FIG. 13(A)  showing the collision positions of the hard ball, and  FIG. 13(B)  showing a measurement condition; 
           [0043]      FIG. 14  is a graph showing ball speeds immediately after a hard ball is caused to collide with a position corresponding to a “dead center” 150 mm distant from the distal end of bat by the experimental method shown in  FIG. 13 ; and 
           [0044]      FIG. 15  is graphs showing ball speeds immediately after a hard ball is caused to collide with a bat by the experimental method shown in  FIG. 13 ,  FIG. 15(A)  showing the case where the collision position is 250 mm distant from the distal end of bat, and  FIG. 15(B)  showing the case where the collision position is 50 mm distant from the distal end of bat. 
       
    
    
     DESCRIPTION OF SYMBOLS 
       [0000]    
       
           11  baseball bat 
           12  reinforcing core 
           13  outer peripheral material 
           21  base material 
           22  laminated body (composed of reinforcing core and bamboo pieces) 
           23  laminated body (composed of bamboo pieces only) 
           24  bamboo piece 
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0052]      FIG. 1  is views showing a construction example of a baseball bat in accordance with the present invention, in which a reinforcing core has a square cross section,  FIG. 1(A)  being a front view and a left side view,  FIG. 1(B)  being a sectional view taken along the line B-B of  FIG. 1(A) ,  FIG. 1(C)  being a sectional view taken along the line C-C of  FIG. 1(A) , and  FIG. 1(D)  being a sectional view taken along the line D-D of  FIG. 1(A) . A baseball bat  11  in accordance with the present invention has a construction such that a reinforcing core  12  having a square cross section, which is arranged in the center so as to penetrate both ends of the bat, and an outer peripheral material  13  consisting of bamboo pieces  24  is provided on the outside of the reinforcing core  12  so as to surround the reinforcing core  12 . In  FIG. 1 , natural hickory is used as the reinforcing core  12 . This reinforcing core  12  is made by sawing raw timber so as to have a square cross section having one side length of 16 mm, and penetrates both ends continuously as shown in the sectional view of  FIG. 1(B) . 
         [0053]    The outer peripheral material  13  covering the outside of the reinforcing core  12  is formed by bonding a large number of bamboo pieces  24 . Each of the bamboo pieces  24  is made by tearing a felled bamboo tree in the fiber direction into a slender rectangular shape. A slender rectangular section hatched in  FIG. 1  represents one bamboo piece  24 . The cross section of one bamboo piece  24  has a long side of about 15 mm and a short side of about 5 mm: however, these dimensions naturally vary. Since the bamboo tree inherently has a cylindrical shape, the bamboo piece  24  is curved strictly. However, since the width of each bamboo piece  24  is far smaller than the diameter of the original bamboo tree, the bamboo piece  24  can be handled as a flat plate. As shown in the C-C sectional view, the adjacent bamboo pieces  24  adhere closely to each other without a gap, and moreover the reinforcing core  12  and the bamboo piece  24  also adhere closely to each other, so that the reinforcing core  12  and the bamboo pieces  24  are integrated as a whole. The bamboo piece  24  arranged at the outermost periphery is arranged so that the lengthwise direction thereof coincides with the radial direction of bat as far as possible. In the hitting part such as the C-C cross section, since the diameter is large, the area ratio of the reinforcing core  12  is naturally low, whereas in the grip part such as the D-D cross section, since the diameter is small, the area ratio of the reinforcing core  12  increases. Therefore, in the grip part, the effect of the reinforcing core  12  is achieved efficiently. 
         [0054]      FIG. 2  shows a cross section of a base material  21  for the baseball bat  11  shown in  FIG. 1 ,  FIG. 2(A)  showing a state in which constituent elements are separated, and  FIG. 2(B)  showing a state in which all of the constituent elements are integrated. The cross-sectional shape of the base material  21  is the same at every place, being a square shape having one side of about 70 mm. The base material  21  is not manufactured by adhering the individual bamboo pieces  24  to each other at a time, but it is manufactured as described below. First, laminated bodies  23  are manufactured by adhering the broad surfaces of the individual bamboo pieces  24  to each other, and laminated body  22  is manufactured by adhering the bamboo pieces  24  to both right and left sides of the reinforcing core  12 . Next, an adhesive is applied to the side surfaces of the individual laminated bodies  22  and  23 , and the plurality of laminated bodies  22  and  23  are integrated. Finally, the base material  21  as shown in  FIG. 2(B)  is completed. Before the base material  21  is manufactured, the surfaces of the laminated bodies  22  and  23  are sometimes scraped to prevent poor bonding. 
         [0055]      FIG. 3  shows a construction example of the baseball bat  11  in which the reinforcing core  12  differs from that of  FIG. 1 .  FIG. 3(A)  is a front view and a left side view,  FIG. 3(B)  is a sectional view taken along the line B-B of  FIG. 3(A) ,  FIG. 3(C)  is a sectional view taken along the line C-C of  FIG. 3(A) , and  FIG. 3(D)  is a sectional view taken along the line D-D of  FIG. 3(A) . The reinforcing core  12  is not limited to one having a square cross section as shown in  FIG. 1 . In  FIG. 3 , the reinforcing core  12  has a rectangular cross section having a long side of 36 mm and a short side of 10 mm. In this embodiment, since the long side of the reinforcing core  12  is larger than the diameter of the grip part, the side surfaces of the reinforcing core  12  are exposed to the outside as shown in  FIG. 3(D) . The cross section of the reinforcing core  12  has an elongated circular shape in this location only. 
         [0056]      FIG. 4  shows a cross section of the original base material  21  of the baseball bat  11  shown in  FIG. 3 .  FIG. 4(A)  shows a state in which constituent elements are separated, and  FIG. 4(B)  shows a state in which all of the constituent elements are integrated. In  FIG. 4 , the reinforcing core  12  and the surroundings thereof are different from those of  FIG. 2 , while other configurations are the same as those of  FIG. 2 . However, since the reinforcing core  12  has a rectangular cross section, the characteristics such as rigidity differ depending on the direction. 
         [0057]      FIG. 5  shows an outline at the time when the flexural rigidity of the baseball bat in accordance with the present invention is measured. As shown in  FIG. 5 , metallic support points were arranged at an interval of 600 mm, the bat center was caused to coincide with the center between the support points, and a concentrated load was applied to the center, by which the load and displacement were measured. For the measurement, a testing device (Autograph AG-2000B manufactured by Shimazu Corporation) owned by Human Life Technology Research Institute in Toyama Industrial Technology Center was used, and the load was applied so that the displacement per minute was 10 mm. All of the bats used for the test had a unified outline shape and an overall length of 840 mm. The measurement was made after the bat was left in a thermostatic chamber of 20 degrees Centigrade for more than a full one day. The measurement results are given in  FIGS. 6 to 9 . 
         [0058]      FIG. 6  shows the measurement results of the flexural rigidity of a baseball bat used conventionally. Before the measurement of the bat in accordance with the present invention, the measurement of wooden bats and bamboo bats was made for comparison. For the wooden bats, one bat using serrata grown in Hokkaido (expressed as Serrata in the figure) and three bats using maple grown in North America (expressed as Maple No. 1, 2 and 3) were used. Also, for the bamboo bats, three bats using bamboo felled in Japan (expressed as Bamboo laminate No. 1, 2 and 3) were used. The abscissas of the figure represent displacement at the position to which the load is applied, and the ordinates thereof represent load. The measurement was continued until some rupture occurs on the bat. 
         [0059]    As shown in  FIG. 6 , the rigidity of wooden bat is apparently higher than that of the bamboo bat. For all of the four wooden bats, in order to produce displacement of about 10 mm, a load of about 1500 N must be applied. In contrast, for the bamboo bats, though there are some variations, in order to produce displacement of about 10 mm, a load of about 1000 N has only to be applied. It is found that the rigidity of bamboo bat is about two-thirds that of the wooden bat. However, the bamboo bat still has properties of natural bamboo, and ruptures after the displacement exceeds 30 mm, which reveals that the flexibility of bamboo bat is high. Thus, there is a difference in rigidity between wooden bat and bamboo bat, and it is thought that this difference exerts an influence on the flying distance of batted ball and the like. 
         [0060]      FIG. 7  shows the measurement results of the flexural rigidity of the baseball bats in accordance with the present invention. In this baseball bat, hickory is used as a material for the reinforcing core, and the reinforcing core has a square cross section having one side of 16 mm. The broken lines in the figure indicate the measurement results of a total of seven bats shown in  FIG. 6 . All of the three bats shown in  FIG. 7  have rigidity lower than that of the wooden bat but higher than that of the bamboo bat, and plot curves between the wooden bat and the bamboo bat. The load at the time when rupture occurs is equivalent to that of the wooden bat. However, this method of measurement does not necessarily reflect the actual load in case of rupture because the application point of load or the like differs from the actual batting. 
         [0061]      FIG. 8  shows the measurement results of the flexural rigidity of the baseball bats in accordance with the present invention. In this baseball bat, hickory is used as a material for the reinforcing core, the reinforcing core has a rectangular cross section having a long side of 36 mm and a short side of 10 mm, and the long side is in agreement with the load direction. The broken lines in the figure indicate the measurement results of a total of seven bats shown in  FIG. 6 . By arranging the reinforcing core in this manner, the geometrical moment of inertia is increased significantly, so that all of these three bats plot curves similar to those of the wooden bat. Therefore, when a bat in which such a reinforcing core having a rectangular cross section is incorporated is used, and the long side of reinforcing core is put in agreement with the direction of a thrown ball, batting can be expected to be similar to that of the wooden bat. 
         [0062]      FIG. 9  also shows the measurement results of the flexural rigidity of the same baseball bats as those shown in  FIG. 8 . However, in this baseball bat, the short side of reinforcing core is put in agreement with the load direction. Therefore, although the rigidity decreases as compared with  FIG. 8 , all of the three bats plot curves between wooden bat and bamboo bat, and the rigidity thereof is higher than that of the bamboo bat. The broken lines in the figure indicate the measurement results of a total of seven bats shown in  FIG. 6 . 
         [0063]      FIG. 10  shows an outline of experiment at the time when the flexural rigidity of the baseball bat  11  in accordance with the present invention is measured under a condition close to the actual batting. As shown in  FIG. 10 , the grip part of the baseball bat  11  is fixed by using a vise, and a concentrated load is applied to a position 150 mm distant from the distal end of bat. This position is a singular point called a “dead center” or a “sweet spot”. For the measurement, the testing device (Autograph AG-2000B manufactured by Shimazu Corporation) owned by Human Life Technology Research Institute in Toyama Industrial Technology Center was used, and the load was applied so that the displacement per minute was 10 mm. In addition to the load at that time, displacement at the application point of load was recorded continuously. However, the maximum load was limited to 2000 N, and the measurement of all bats was finished within the range of elastic deformation. All of the bats used for the experiment had a unified outline shape and an overall length of 840 mm. The measurement was made after the bat was left in an environment of 10 degrees Centigrade for more than a full one day. The measurement results are given in  FIGS. 11 and 12 . 
         [0064]      FIG. 11  shows the measurement result of the flexural rigidity of the baseball bat  11  having a reinforcing core of a square cross section (expressed as Square reinforcing core in the figure), the measurement result being obtained by the experimental method shown in  FIG. 10 . In this baseball bat, hickory having a square cross section having one side of 16 mm is used as a material for the reinforcing core. For comparison, the measurement results of wooden bats and bamboo bat are also shown. For the wooden bats, a bat using serrata grown in Hokkaido (expressed as Serrata in the figure) and a bat using maple grown in North America (expressed as Maple in the figure) were used. Also, for the bamboo bat, a bat using bamboo grown in Japan (expressed as Bamboo laminate in the figure) was used. According to  FIG. 11 , it is found that the bat having a reinforcing core has characteristics apparently different from those of the bamboo bat, and has rigidity similar to that of the wooden bat. 
         [0065]      FIG. 12  shows the measurement results of the flexural rigidity of the baseball bat  11  having a reinforcing core of a rectangular cross section, the measurement results being obtained by the experimental method shown in  FIG. 10 . In this baseball bat, hickory having a rectangular cross section having a long side of 36 mm and a short side of 10 mm is used as a material for the reinforcing core. For comparison, the measurement results of wooden bats and bamboo bat, which are the same as those shown in  FIG. 11 , are also shown. As shown in  FIG. 12 , in the case where a load is applied in the long side direction of the reinforcing core, the effect of reinforcing core is achieved remarkably, and the rigidity is increased as compared with the wooden bat. In contrast, in the case where a load is applied in the short side direction of the reinforcing core, the rigidity is decreased as compared with the wooden bat, but is improved as compared with the bamboo bat. 
         [0066]      FIG. 13  shows an outline of experiment at the time when a hard ball is caused to collide with the baseball bat  11  in accordance with the present invention or the like and the speed of the hard ball immediately after the collision is measured.  FIG. 13(A)  shows the collision positions of the hard ball, and  FIG. 13(B)  shows a measurement condition. As shown in  FIG. 13 , the grip part of the baseball bat  11  is fixed by a total of four rollers, and the hard ball is caused to collide with the distal end part of the bat. The hard ball is a hard ball officially recognized by Japan High School Baseball Federation, and a predetermined initial speed is given to the hard ball by utilizing compressed air. Also, to measure the ball speeds immediately before and after the collision, an optical speed sensor is arranged. The experiment was conducted in an environment of a centigrade temperature of about 23 degrees and a relative humidity of 40%. As in the cases shown in  FIGS. 11 and 12 , for all of the bats, as shown in  FIG. 13(A) , at each of three positions 50 mm, 150 mm, and 250 mm distant from the distal end of bat, measurement was made five times, and the average value was calculated. The measurement results are shown in  FIGS. 14 and 15 . To absorb variations in speed immediately before the collision, as shown in  FIG. 13(B) , the actual speeds before and after the collision were measured, and then the speed immediately before the collision was converted to 100 km/h, by which the value shown in  FIGS. 14 and 15  was calculated based on this conversion. 
         [0067]      FIG. 14  shows ball speeds immediately after a hard ball is caused to collide with a position corresponding to the “dead center” 150 mm distant from the distal end of bat by the experimental method shown in  FIG. 13 . As shown in  FIG. 14 , in addition to the baseball bats  11  having the reinforcing core (expressed as Reinforcing core in the figure) in accordance with the present invention, both of the wooden bats (expressed as Serrata and Maple in the figure) have a reflection speed in a relatively narrow range of 29.0 to 31.0 km/h, whereas the bamboo bat (expressed as Bamboo laminate in the figure) has a slightly low reflection speed of 27.9 km/h. For the baseball bat incorporating the reinforcing core having a rectangular cross section, the experiment was conducted in both of the case where a ball is caused to collide in the long side direction and the case where a ball is caused to collide in the short side direction. As seen from  FIG. 14 , the baseball bat  11  in accordance with the present invention has performance equivalent to that of the wooden bat in the case where a ball is batted at the dead center, and apparently has superiority over other balls. In this experiment, since the bat is fixed, the speed immediately after the collision is about 30% of the speed immediately before the collision. Actually, however, the speed due to swing is added. 
         [0068]      FIG. 15  shows ball speeds immediately after a hard ball is caused to collide with a bat by the experimental method shown in  FIG. 13 .  FIG. 15(A)  shows the case where the collision position is 250 mm distant from the distal end of bat, and  FIG. 15(B)  shows the case where the collision position is 50 mm distant from the distal end of bat. In both of the experiments shown in  FIG. 15 , the collision position deviates from the dead center, reflecting a condition that long hit cannot be anticipated in the actual scene, so that the speed immediately after the collision is lower as compared with the case of  FIG. 14 . Moreover, the difference in speed between bats is small. It is presumed, under such a condition, that the bat performance does not differ greatly even if any bat is used.
There is provided a baseball bat that has performance similar to that of the conventional wooden bat and moreover is excellent in terms of cost and resource conservation. For this baseball bat 11, a reinforcing core 12 having a rectangular cross section is arranged in the center of the cross section thereof continuously so as to penetrate both end thereof, bamboo pieces 24 are laminated around the reinforcing core 12, and an adhesive is caused to permeate into the contact surface between the reinforcing core 12 and the bamboo pieces 24 evenly under a pressure, by which firm bonding can be attained. In addition, by using a material that is hard and has a large specific gravity as compared with bamboo, such as hickory or reinforced wood, as the reinforcing core 12, the flexural rigidity is enhanced as a whole, and the weight is increased. Therefore, the characteristics and feel in use are improved as compared with the conventional bamboo bat, and the baseball bat 11 can be used as a substitute for a wooden bat. Bamboo, hickory, and reinforced wood have no problem in terms of resource conservation, can reduce the product price, and can lower the consumption of scarce resources such as serrata.