DESCRIPTION STAPLER TECHNICAL FIELD [0001] The invention relates to a stapler. More particularly, it relates to a stapler that passes a pair of legs of a staple, which is constituted of a crown portion and the legs extending from both ends of the crown portion, through a stack of paper based on any predetermined clinching force and clinches the legs of staple to staple the stack of paper. BACKGROUND ART [0002] Among cases where stapling a stack of paper, a stapler has often driven the staple into the stack of paper to staple the stack of paper until now. This stapler contains a clincher arm, a magazine, a pusher, a driver arm, a driver, a handle, and the like. The pusher pushes a strip of staples loaded in the magazine to bias it onto a staple-pushing-out slot of the magazine. [0003] When any clinching force is applied to the clincher arm, the magazine, and the driver arm, which share a main shaft with their shaft-receiving portions, through the handle, the driver provided at a forward end of the driver arm drives a top staple of the strip of staples loaded in the magazine out of the staple-pushing-out slot. For example, a forward end of the driver is formed flat and pushes the whole crown portion of the staple to drive it out. Since the driven staple comes into contact with a clincher provided with a forward end of the clincher arm, the legs thereof are clinched round or flat. [0004] When this operation is performed with the stack of paper being clipped by the magazine and the clincher, the staple, the legs of which are clinched round or flat, can staple this stack of paper. [0005] In relation to such a conventional example, Japanese Patent Application Publication No- H09-085644 has disclosed a stapler on page 3 and FIG. 2 thereof in which a forward end of pushing-out portion for pushing the staple out is formed so as to be concave. According to this stapler, both ends of the concave forward end of the pushing-out portion contact portions of the staple over the legs thereof when pushing the staple out. This enables any force applied to the staple by the pushing-out portion to be concentrated on the legs of the staple so that it can be more increased than that of a past case. DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0006] By the way, according to the stapler concerning the conventional example, by applying any predetermined clinching force, the flat-formed forward end of the driver pushes the whole crown portion of the staple to drive the staple out. This causes any pushing force for pushing the whole crown portion thereof to spread over the crown portion, so that if a stack of paper in which there is a large number of sheets of paper (around its maximum stackable sheets) is particularly stapled, a problem occurs such that buckling is easily generated by failing to concentrate the pushing force on the legs of the staple. [0007] According to the staple disclosed in Japanese Patent Application Publication No. H09-085644, which has coped with this problem, the buckling is prevented by concentrating the pushing force to the legs by the push-out portion, the forward end of which is formed so as to be concave. It is, however, difficult to prevent the buckling merely by concentrating the pushing force to the legs. This is because a large amount of pushing force is applied to the legs if postures of the legs are inclined even slightly against a sheet of paper. MEANS FOR SOLVING THE PROBLEMS [0008] This invention solves such problems relating to the conventional examples and has an object to provide a stapler that is capable of concentrating the clinching force onto the legs of staple and preventing the buckling of the staple from occurring. [0009] In order to solve the above-mentioned problems, a stapler according to the invention passes a pair of legs of a staple, which is constituted of a crown portion and the legs extending from both ends of the crown portion, through a stack of paper based on any predetermined clinching force and clinches the legs thereof to staple the stack of paper, and is characterized in that the stapler comprises a magazine that loads a staple therein, the magazine containing a staple-pushing-out slit at an end thereof, a driver arm that contains a driver which comes into contact with the crown portion of the staple positioned on the staple-pushing-out slit at an end thereof and drives the staple based on the predetermined clinching force, and a staple-pushing member that is mounted near the staple-pushing-out slit and pushes the crown portion of the staple driven out by the driver arm against a front inner wall of the magazine wherein the driver of the driver arm includes a projection that comes into contact with a portion of the staple over the legs thereof, the staple being pushed against the front inner wall of the magazine by the staple-pushing member. [0010] In the stapler according to the present invention, the projection of the driver of the driver arm drives the staple with the projection contacting the portion of the staple over the legs thereof, the staple being pushed against the front inner wall of the magazine by the staple-pushing member. This enables clinching force to be transferred to the legs through the projection of the driver, thereby concentrating the clinching force on the legs. [0011] Furthermore, since the staple is pushed by the staple-pushing member, the buckling may be prevented from occurring in the legs. Alternatively, if the driver is provided with projections at both ends thereof, the driver drives the staple with the crown portion thereof being curved by the projections, so that reaction by the stack of paper to the legs can be set off. Accordingly, es the staple may receive the reaction borne to it suitably, it is possible to provide the stapler that has high resistance to the buckling. BRIEF DESCRIPTION OF DRAWINGS [0012] [FIG. 1] is a sectional view of an embodiment of a stapler 100 according to the invention for showing a configuration example thereof; [FIG. 2] is a perspective view of a strip of staples 300 for showing a configuration example thereof; [FIG. 3A] is a top view of the strip of staples 300 for showing a configuration example thereof; [FIG. 3B] is a front view of the strip of staples 300 for showing a configuration example thereof; [FIG. 4A] is a front view of a staple 31 concerning No. 10 based on Japanese Industrial Standard (hereinafter, referred to as "JIS") S6036 concerning a conventional example; [FIG. 4B] is a front view of a staple 32 concerning No. 3 based on JIS S6036 concerning a conventional example; [FIG. 4C] is a front view of a staple 30 according to the invention for showing a configuration example thereof; [FIG. 5A] is a front view of the staple 30 for showing a basic state thereof; [FIG. 5B] is a front view of the staple 30 for showing a bent state thereof; [FIG. 6A] is a front view of a preferable staple 30 for showing a basic state thereof; [FIG. 6B] is a front view of the preferable staple 30 for showing a bent state thereof; [FIG. 7] is a perspective view of an assembly of a magazine 3 and parts relating thereto for showing an assembled example; [FIG. 8] is an exploded perspective view of the assembly of the magazine 3 and the parts relating thereto; [FIG. 9A] is a top view of the magazine 3 for showing an attachment of a staple-pushing member 40 thereto; [FIG. 9B] is a sectional view of the magazine 3, taken on line Xl-Xl shown in FIG. 9A, for showing the attachment of the staple-pushing member 40 thereto; [FIG. 10A] is a sectional view of the magazine 3, taken on line Yl-Yl shown in FIG. 9A, for showing the attachment of the staple-pushing member 40 thereto; [FIG. 10B] is an enlarged view of the magazine 3 for showing a part of the magazine 3 enclosed by broken lines shown in FIG. 10A; [FIG. 11A] is a top view of the magazine 3 for showing an attachment of a staple guide 50 thereto; [FIG. 11B] is a sectional view of the magazine 3, taken on line X2-X2 shown in FIG. 11A, for showing the attachment of the staple guide 50 thereto; [FIG. 12A] is a sectional view of the magazine 3, taken on line Y2-Y2 shown in FIG. 11A, for showing the attachment of the staple guide 50 thereto; [FIG. 12B] is an enlarged view of the magazine 3 for showing a part of the magazine enclosed by broken lines shown in FIG. 12A; [FIG. 13] is a perspective view of a pusher 6 for showing a configuration example thereof; [FIG. 14A] is a top view of the magazine 3 for showing an attachment of the pusher 6 thereto; [FIG. 14B] is a top view of a pusher band 6c to be assembled to the pusher 6 for showing a configuration example thereof; [FIG. 14C] is a sectional view of the magazine 3, taken on line X3-X3 shown in FIG. 14A, for showing the attachment of the pusher 6 thereto; [FIG. 15A] is a side view of another pusher 61 for showing a configuration example thereof; [FIG. 15B] is the other side view of the above-mentioned another pusher 61 for showing a configuration example thereof; [FIG. 16A] is a whole perspective view of a driver 5 for showing a configuration example of the driver 5; [FIG. 16B] is an enlarged front view of an important portion of the driver 5 for showing a configuration example of a driving portion 5d of the driver 5; [FIG. 17] is a perspective view of an assembly of the driver 5 and the magazine 3; [FIG. 18A] is an elevation for showing an operation example (part one) of the driver 5 and the staple guide 50; [FIG. 18B] is an elevation for showing the operation example (part two) of the driver 5 and the staple guide 50; [FIG. 18C] is an elevation for showing the operation example (part three) of the driver 5 and the staple guide 50; [FIG. 18D] is an elevation for showing the operation example (part four) of the driver 5 and the staple guide 50; [FIG. 19A] is a side elevation for showing an operation example (part one) of the driver 5 and the staple guide 50; [FIG. 19B] is a side elevation for showing the operation example (part two) of the driver 5 and the staple guide 50; [FIG. 19C] is a side elevation for showing the operation example (part three) of the driver 5 and the staple guide 50; [FIG. 19D] is a side elevation for showing the operation example (part four) of the driver 5 and the staple guide 50; [FIG. 20A] is a side elevation for showing an operation example (part one) of the driver 5 and the staple-pushing member 40; [FIG. 20B] is a side elevation for showing the operation example (part two) of the driver 5 and the staple-pushing member 40; [FIG. 20C] is a side elevation for showing the operation example (part three) of the driver 5 and the staple-pushing member 40; [FIG. 20D] is a side elevation for showing the operation example (part four) of the driver 5 and the staple-pushing member 40; [FIG. 21] is a perspective view of an assembly of a clincher arm 1, a clincher arm cover 2, and af clincher buffer 60 for showing a fitting example of the clincher buffer 60 thereto; [FIG. 22] is an exploded perspective view of the assembly of the clincher arm 1, the clincher arm cover 2, and the clincher buffer 60 for showing a configuration example -jf the clincher buffer 60; [FIG. 23A] is a top view cf the clincher arm 1 for showing an attachment example of the clincher buffer 60 thereto; [FIG. 23B] is a sectional view of the clincher arm 1 and the clincher buffer 60, taken on line X4-X4 of FIG. 23A, for showing the attachment example of the clincher buffor 60 thereto; [FIG. 23C] is an elevation of the clincher arm 1 and the clincher buffer 60, shown from a direction along an arrow Y4 of FIG. 23A; [FIG. 24] is a perspective view of an assembly of a handle 8, a handle cover 9, and a handle buffer 64 for showing a fitting example of the handle buffer 64 thereto; [FIG. 25] is an exploded perspective view of the assembly of the handle 8, the handle cover 9, and the handle buffer 64 for showing a configuration example cf the handle buffer 64; [FIG. 26] is a diagram for showing the 'configuration example of the handle buffer 64 and illustrating an attachment example of the handle buffer 64 to the handle 8; [FIG. 27A] is a bottom cie\.' of the handle 8 to which the handle buffer 64 is attached; [FIG. 27B] is a sectional view taken on line X5-X5 of FIG. 27A; [FIG. 27C] is an elevation shown from a direction along an arrow Y5 of FIG. 27A; [FIG. 28A] is a sectional view of an important portion of the stapler 100 in which coil springs 10, 20c, and 20d are installed; [FIG. 28B] is a perspective view of the coil spring 10 for showing a configuration example thereof; [FIG. 29] is an exploded perspective view of the coil spring 10; [FIG. 30] is a perspective view of a back cover 15 for showing a configuration example thereof; [FIG. 31A] is a side view of the back cover 15; [FIG. 31B] is a front view of the back cover 15; [FIG. 31C] is a top view of the back cover 15; [FIG. 32A] is a perspective view of an assembly of a magazine 3 and the back cover 15, shown from above, for showing a fitting example of the back cover 15 thereto; ; [FIG. 32B] is a perspective view of the assembly of the magazine 3 and the back cover 15, shown from below, for showing the fitting example of the back cover 15 thereto; [FIG. 33A] is a bottom view of the assembly of the magazine 3 and the back cover 15 for showing the fitting example of the back cover 15 thereto; [FIG. 33B] is a sectional view taken on line X6-X6 of FIG. 33A for showing the fitting example of the back cover 15 thereto; [FIG. 34A] is a side view of the assembly of the magazine 3 and the back cover 15 for showing the fitting example of the back cover 15 thereto; [FIG. 34B] is a sectional view taken on line X7-X7 of FIG. 34A for showing the fitting example of the back cover 15 thereto; [FIG. 35A] is a sectional view of the stapler 100 for showing a standby state thereof; and [FIG. 35B] is a sectional view of the stapler 100 for showing a state thereof where it staples attack of paper. ... * '' BEST MODE FOR CARRYING OUT THE INVENTION [0013] i The following will describe embodiments of a stapler according to the invention and a staple therefor with reference to the attached drawings. [0014] A configuration example of an embodiment of the stapler 100 according to the invention will be described with reference to FIG. 1. The stapler 100 shown in FIG. 1 is a small-sized portable stapler. It is supposed that for this stapler 100, a strip of staples 30, as shown in FIG. 2, each staple having a size larger than that of a staple of No. 10 based on JIS S6036 and smaller than that of a staple of No. 3 based on JIS S6036, namely, an intermediate sized staple 30 between the staple of No. 10 and the staple of No. 3 is suitably used. [0015] The stapler 100 has a function to clinch the staple 30 based on any predetermined clinching force. This stapler 100 has a clincher arm 1. The clincher arm 1 is made of a metal plate and is formed by bending the metal plate so as to have a bottom and both sides. The clincher arm 1 constitutes a base portion of a main body of the stapler 100. A clincher portion Is is mounted on a forward end of the clincher arm 1. This clincher portion Is has a clincher Ih, on an upper end surface of which grooves are formed, and a spring, not shown, for biasing the clincher lh upwards. The grooves formed on the upper end surface of the clincher lh function as bending the legs 30b (see FIG. 2) of the staple 30 round or flat. It is to be noted that the clincher lh having a configuration as shown in the embodiment is indicated as a preferable case where the legs 30b, 30b of the staple 30 are bent flat. [0016] Further, this clincher lh is formed so as to have such a linear bending structure that the grooves are formed linearly on the same line and the legs 30b are fitted along the grooves and bent in the same line (hereinafter, referred to as "in-line clincher system") or such a tips-not-facing bending structure that the grooves are formed on two lines and the legs 30b are bent by the grooves not so as to face the tips of the legs 30b to each other (hereinafter, referred to as "by-pass clincher system") . It is to be noted that in this embodiment, the in-line clincher system is applicable to the clincher lh. [0017] The clincher lh biased upwards is positioned so as to stay within an opening in a clincher guide portion 20. In this embodiment, an upper end surface of the clincher lh is set so as to become stationary at a position (upper dead point) that is approximately equal to the opening of the clincher guide portion 20. [0018] This clincher guide portion 20 is mounted on the clincher arm 1 and once maintains a space'between the magazine 3 and the clincher lh when the staple 30 is driven out of a staple-pushing-out slit 12 of the magazine 3. The clincher guide portion 20 is then pushed down by the magazine 3 based on an operation of an operation lever 5a attached to a driver arm 4. [0019] This clincher guide portion 20 is constituted of a clincher guide 20a, a sliding member 20b, and coil springs 20c, 20d. The clincher guide 20a has an engaging claw, not shown, at a rear end of the clincher guide 20a, and the engaging claw rotatably engages an edge of an opening portion, not shown, formed in the clincher arm 1. This clincher guide 20a is biased upwards by the coil spring 20c. On a forward end portion of the clincher guide 20a, contacting column 20e extending from a main body of the clincher guide 20a downwards in a nearly vertical direction is provided. This contacting column 20e is supported by a contact with a supporting portion 20f of the sliding member 20b, which is slidably attached. This enables the clincher guide 20a to be locked by the sliding member 20b. This sliding member 20b is biased forward by the coil spring 20d. Thus, the clincher guide portion 20 is configured. [0020] A cover for clincher arm (hereinafter, referred to as "clincher arm cover 2") , which is mads of synthetic resin and to which any design is applied, covers a rear surface side of the clincher arm 1. The magazine 3 is rotatably mounted on this clincher arm 1. The magazine 3 has a main frame 3a (see FIG. 7) having a predetermined shape. The magazine 3 contains the staple-pushing-out slit 12 formed on a forward end portion of its main frame 3a and holes for supporting the main shaft (hereinafter, referred to as "holes 3b, 3c") formed on the other portion thereof as well as loads a strip of staples 300. [0021] The main shaft 21 is inserted into the holes 3b, 3c of the clincher arm 1 at a rear side of the magazine 3. The magazine 3 may be designed so as to load merely one or two strips of the staples 300, one strip being constituted of 50 staples (see FIG. 2) . [0022] In the magazine 3, the pusher 6 is put on a spring guide 6a so as to be slidably guided, and pushes the strip of staples 300 loaded in the magazine 3 toward the staple-pushing-out slit 12 formed on a forward end of the magazine 3. [0023] A coil spring 10 stands between a rear side of the magazine 3 and a rear side of the clincher arm 1. This coil spring 10 operates so as to keep a predetermined space between the clincher arm 1 and the magazine 3 in order to allow the stack of paper to intervene in the space between the clincher arm 1 and the magazine 3. [0024] To the above-mentioned main shaft 21, the driver arm 4 is rotatably attached. This driver arm 4 has a main frame. This main frame is formed by bending a metal plate so as to have a top surface and both side surfaces. The driver arm 4 has the driver 5 at an end (forward end) thereof and holes for receiving the main shaft 21 (a first driving-shaft-supporting portion) at the other end (rear end) thereof. [0025] The driver 5 has an almost L-shape having elasticity. A main body of the driver 5 is fixed closely to the top surface of the main frame of the driver arm and a forward portion of the driver 5 is fixed so as to be nearly perpendicular to the top surface of the main frame of the driver arm. Further, a rear portion of the driver 5 is formed so as to be curved away from the top surface of the main frame of the driver arm 4 so that a curved elastic postion 5c (see FIG. 16A) contacts a staple cover 7 mounted on the magazine 3. This enables a space between the driver arm 4 and the magazine 3 to be kept constant under standby state of the stapler 100. It is to be noted that the staple cover 7 shares the main shaft 21 by its rear end and covers the strip of staples 300 loaded in the magazine 3. [0026] When driving the driver arm 4 in a direction (counterclockwise) for clinching the staple 30, the rear portion of the elastic driver 5 approaches to the driver arm 4 ;and the driver 5 advances within the magazine 3. This driver 5 comes into contact with an upper portion of a top staple 30 of the strip of staples 300 pushed by the pusher 6 and pushes the upper portion of the top staple 30 down by pushing-down force applied to the driver arm 4 r.o drive the top staple 30 to a stack of paper. [0027] The staple cover 7 is mounted on the magazine 3 inside the main frame of the driver arm 4 and pushes down the strip of staples 300 loaded in the magazine 3. The staple cover 7 is rotatably attached to the main shaft 21. In this embodiment, the staple cover 7 has a U-shaped end, which is spread out over the main shaft 21, and is rotatably supported by the main shaft 21. [0028] A handle 8 is provided over the driver arm 4 and is manipulated so as to apply any clinching force to the driver 5 of the driver arm 4. The handle 8 has a main frame 8a (see FIG. 25) formed by bending a metal plate so as to have a top surface and both side surfaces. [0029] A handle cover 9 is provided on the handle 8 zo cover a surface of the main frame 8a of the handle 8. The handle cover 9 is constituted of a molded frame which is made of synthetic resin and to which any design is applied, similar to a cas of the clincher arm cover 2. [0030] The handle 8 and the handle cover 9 rotatably pivot around a second driving-shaft-supporting portion (hereinafter, referred to as "connection axis 22"), which is positioned above the main shaft 21, at their rear portions. For example, the connecting axis 22 is constituted of projected axes 22a, 22a (see FIG. 23B) which are provided as bosses on side surfaces of the sheet metal of the clincher arm 1 by a burring process or the like, and axis-receiving portions 22b, 22b (see FIG. 25) , each of which is recessed as U-shaped on the side surface of the handle 8. [0031] Furthermore, there is a point of application "q" for pushing the driver arm 4 down on the way from' the connection axis 22 of the handle 8 and the handle cover 9 to the forward end of the magazine 3 (to a direction of a staple-pushing-out slit). An axis for the point of application (hereinafter, referred to as "application axis 23"), which may be freely engaged with the handle 8 and the driver arm 4, is provided at the point of application "q". [0032] Thus, when the connection axis 22 is positioned at an upper position than that of the main shaft 21 that is shared by the magazine 3 and the driver arm 4, the driver arm 4*is pushed down at the point of application "q" with the connection axis 22 acting as a fulcrum if any force is applied to the forward end of each of the handle 8 and the handle cover 9, which is set as a point "p" where the force is to be applied to the stapler. Thus, the stapler 100 can staple the stack of paper by a staple 30 with less pushing-down force that is applied to the point "p" of the handle cover 9 (using a mechanism for multiplication of force achieved by a lever-fulcrum relationship). [0033] A back cover 15 is provided on a rear end portion of the stapler 100, which is constituted of the main shaft 21 of magazine 3, the driver arm 4, and the staple cover 7 and the connection axis 22 of the handle 6 and the handle cover 9, and the like. This back cover 15 can prevent any foreign matter from entering into an opening formed by any rear parts of the stapler 100 such as the clincher arm cover 2, the handle cover 9 and the like, and can realize a stapler 100 having a good appearance in an external design. [0034] The following will describe an operation of the stapler 100. When the handle 8 is pushed down through the upper side of the handle cover 9, the handle 8 rotates around the connection axis 22 to push the application axis 23 down. When the application axis 23 is pushed down, the driver arm 4 and the magazine 3 rotate around the main shaft 21 and a forward end of the magazine 3 contacts a forward end side of the clincher guide 20a. When the handle 8 is further pushed down in this state, the handle 8 and the driver arm 4.rotate with positions of the magazine 3 and the clincher guide 20a are fixed and then, the driver 5 provided on a forward end of the driver arm 4 drives the top staple 30 of the strip of staples 300 loaded in the magazine 3 to push it out of the staple-pushing-out slit 12 from inside of the magazine 3 to outside. Pushed-out staple 30 descends with the legs 30b being kept on their straight states and pushes the clincher lh down to a bottom dead center thereof against the biasing force by a spring, not shown, for biasing the clincher lh upwards. [0035] Then, when the handle 8 is further pushed down, an operation lever 5a proved on this driver 5 rotates counterclockwise, and the sliding member 20b retreats and is moved rearwards by the forward end of the operation lever 5a. This rearward movement of the sliding member 20b causes to be made free (to its clutch-off state) the contacting column 20e of the clincher guide 20a that has been supported by the supporting portion 20f of the sliding member 20b. The clincher guide 20a is then pushed down and swung through the magazine 3 together with rotations of the handle 8, the driver 5;- and the magazine 3. In this moment, by the rotation of the driver 5 and the clincher lh positioned at the bottom dead center, legs 30b of the staple 30 contacted with the driver 5 are bent flat. Thus, clinching operation by the staple 30 is performed. [0036] The following will describe configurations of the staple 30 and the strip of staples 300. The strip of staples 300 shown in FIG. 2 is constituted of 50 staples that are welded together in series. The strip of staples 300 is loaded in the magazine 3. [0037] The staple 30 is constituted of the crown portion 30a and a pair of legs 30b extending downwards from both ends of the crown portion 30a. [0038] FIG. 3A shows a top surface of the strip of staples 300 shown in FIG. 2 and FIG. 3B shows a front surface of the strip of staples 300 shown in FIG. 2. If it is supposed that length to which an inner width L5 between a pair of the legs 30b shown in FIG. 3B and thickness L4 of each of the legs 30b is added is set as whole length Ll of the crown portion 30a of the staple 30, the whole length Ll of the crown portion 30a is formed so as to be longer than 9.48 mm and shorter than 12.45 mm. Further, it is formed so that the sum of a pair of the lengths L2 of the pair of legs 30b of the staple 30 is not longer than the whole length Ll of the crown portion 30a. Additionally, thickness L3 of the crown portion 30a along a direction of movement thereof is formed so as to be thicker than 0.47 mm and thinner than 0.53 mm. The thickness L4 of each of the legs 30b is formed so as to be thicker than 0.30 mm and thinner than 0.54 mm. [0039] The following will describe comparison examples of the staples 31, 32 and the staple 30 used in this stapler 100 according to the invention with reference to FIGS. 4A through 4C. The staple 31 of No. 10 based on JIS S6036 as shown in FIG. 4A is used with supposing a case where a stack of paper of about 20 sheets of paper is stapled and whole length Lla of the crown portion 31a is regulated so as to be not longer than 9.48 mm and an inner width L5a between a pair of the legs 31b is regulated so as to be not shorter than 8.40 mm. Further, length L2a of each of the legs 31b of the staple 31 is regulated so as to be 4.8 plus or minus 0.2 mm, thickness L4a of each of the legs 31b is regulated so as to be not thinner than 0.30 mm, and thickness of the crown portion 31a along a direction of movement thereof is regulated so as to be 0.50 mm plus or minus 0.03 mm. i" [0040] On the other hand, the staple 32 of No. 3 based on JIS S6036 as shown in FIG. 4B is used with supposing a case where a stack of paper of about 30 sheets of paper is stapled and length Lib of the crown portion 32a is regulated so as to be not longer than 12.97 mm and an inner width L5b, 5b between a pair of the legs 32b is regulated so as to be not shorter than 11.55 mm. Further, lengths L2b of each of the legs 32b is regulated so as to be 6.0 plus or minus 0.2 mm, thickness L4b of each of the legs 32b is regulated so as to be not thinner than 0.45 mm, and thickness of the crown portion 32a along a direction of movement thereof is regulated so as to be 0.70 mm plus or minus 0.03 mm. [0041] The staple 30 shown in FIG. 4C is formed so as to have an intermediate size between the above-mentioned staple 31 of No. 10 and the above-mentioned staple 32 of No. 3. In this embodiment, the crown portion 30a of the staple 30 is formed so as to be longer than the whole length Lla of the crown portion 31a of the staple 31 of No. 10 by difference Ml and shorter than the whole length Lib of the crown portion 32a of the staple 32 of No. 3 by difference M2. [0042] The whole length LI of the crown portion 30a of this staple 30 is longer than the above-mentioned whole length Lla and shorter than the above-mentioned whole length Lib, namely, is set to one within a range Nl (0mm