EXTERIOR MATERIAL FIXING MEMBER BACKGROUND OF THE INVENTION 1. Field of the Invention 5 The present invention relates to a fastening member used for fastening exterior wall boards, such as ceramic siding boards, to a building frame. 2. Description of the Related Art 10 During construction of an exterior wall of a building, a siding board 40 as shown in FIG. 4 that has an underlying rabbeted portion 44 and an overlying rabbeted portion 42 at least at the upper and lower edges thereof is fastened to a building frame using a fastening member (see JP 2001-27031A, for example). FIG. 5 shows a fastening member disclosed in JP 15 2001-27031A. The fastening member 50, which is formed by a sheet metal work process, includes support portions 51a and 51b adapted to abut on a back surface of an upper and a lower building board, respectively, when the boards are fastened, a locking portion 52 formed by successively bending a central portion of the support portion 51, a base plate portion 53 above the 20 support portion 51a, and a bend portion 54 at the lower edge of the support portion 51b. When the fastening member 50 is fixed to the building frame, the base plate portion 53 and the end of the bend portion 54 constitute mount portions via which the fastening member 50 is mounted on the building frame. 25 During construction, an upper board is fastened with the overlying rabbeted portion 42 at the lower edge thereof being locked on the locking portion 52 and the back surface thereof abutting against the support portion 51a. A lower board is fastened with the underlying rabbeted portion 44 at the upper edge thereof being locked on the locking portion 52 and the back 30 surface thereof abutting against the support portion 51b. The fastening 1 member 50 further includes a depressed opening 61 formed in the support portion 51a that has such a depth as to reach the building frame, thereby stabilizing the state of the fastening member 50 attached to the building frame. When the boards are fastened, a space is established between the 5 back surface of the boards and the building frame, the space corresponding to the interval between the face of the base plate portion 53 and the face of the support portion 51, so that ventilation can be ensured. JP 9-302889A discloses a fastening member 70 that is also formed by a sheet metal work process, as shown in FIG. 6. The fastening member 70 10 includes a base plate portion 71 that constitutes a mount portion to be fixed to the building frame, and support portions 72 formed at either side edge of the base plate portion 71 such that they protrude forward while extending longitudinally. The fastening member 70 further includes a locking portion 73 formed laterally between the support portions 72 on either side, with the 15 end of the locking portion 73 located forward of the support portions 72. The locking portion 73 is formed by bending a lower region of the base plate portion 71. The locking portion 73 includes upwardly extending portions 75 formed by bending opposite side portions of the end of the locking portion 73 upward, and a downwardly extending portion 76 formed by bending a center 20 portion of the aforementioned end of the locking portion 73 downward. The upper board is fastened with the overlying rabbeted portion 42 at the lower edge thereof being locked on the locking portion 73 and the back surface thereof abutting against the support portions 72. The lower board is fastened with the underlying rabbeted portion 44 at the upper edge thereof 25 being locked on the locking portion 73 and the back surface thereof similarly abutting against the support portions 72. When the boards are fastened, a space is established between the back surface of the boards and the structural frame, the space corresponding to the height of the support portions 72, so that ventilation is ensured. 30 JP 2005-133504A discloses a fastening member 210 that is also 2 formed by a sheet metal work process, as shown in FIG. 7. The fastening member 210 includes a left-side support portion 212L and a right-side support portion 212R on the left and right sides thereof that are adapted to abut on the back surface of an exterior wall board when the board is fastened. 5 The fastening member 210 further includes in an intermediate region between the left-side and right-side support portions, in sequence from the top: a base plate portion 211 that serves as a mount portion to be fixed to a building frame; a central support portion 212C that is adapted to abut on the back surface of a board when the board is fastened; a locking portion 221 that 10 extends from the lower edge of the central support portion and that is adapted to be locked on the upper edge and the lower edge of respective boards when the boards are fastened; and a lower support portion 212U that is adapted to abut on the back surface of a board when the board is fastened. The fastening member 210 is formed from a single steel sheet. A portion of 15 an upper region of the steel sheet is depressed toward the back surface thereof so that the base plate portion 211 is formed, and a portion of a lower region of the steel sheet is cut and raised toward the front surface thereof so that the locking portion 221 is formed. In this fastening member 210, the locking portion 221 is formed by cutting out a portion of the steel sheet as the 20 material for the fastening member and then utilizing the cut-out steel sheet, and so the fastening member 210 can be entirely formed from a single sheet and therefore can be more lightweight and cost-effective than the conventional fastening members 50 and 70 shown in FIGS. 5 and 6. In the fastening member 50 shown in FIG. 5, since the locking portion 25 52 is formed from a double laminate steel sheet, the fastening member is rather heavy and requires a lot of steel, resulting in higher cost. In the fastening member 70 shown in FIG. 6, the locking portion 73 is formed from a single sheet and is therefore less heavy and less costly. However, since the locking portion 73 is extended from the base plate portion 30 71, which is the mount portion on the building frame, the locking portion 73 3 of the fastening member 70 is longer than those of fastening members such as the fastening member 50 shown in FIG. 5 in which the locking portion 52 is extended from the support portion 51. Thus, the strength of the locking portion 73 is rather unreliable, particularly when a heavy, large-sized board 5 is to be fastened. Furthermore, the fastening member 70 is designed to support the back surface of the board exclusively with the left and right support portions 72. Thus, when adjacent portions of two laterally adjacent boards are to be fastened with a single fastening member 70, the back surface of each board 10 must be supported by one of the two, i.e., left and right, support portions 72, resulting in a stability problem in such a fastening. In the fastening member 210 shown in FIG. 7, a substantially U-shaped portion as seen in a plan view of a lower region 200B of a steel sheet 200 is punched out as shown in FIG. 8, thereby forming an extending 15 portion 202 that serves as the locking portion 221 of the fastening member 210 after processing. The extending portion 202 includes, at the end thereof, three projecting portions 202a, 202b, and 202c that serve as upwardly extending portions 222 or a downwardly extending portion 223 after processing. 20 In order to determine to what wind pressure an exterior wall of a building can endure when a strong wind (wind pressure) acts on the exterior wall, and thereby ascertain the safety, a wind pressure resistance test on an exterior wall panel (Test of wind resistance of exterior walls, General Building Research Corporation of Japan) can be performed. In the above 25 wind pressure resistance test, the performance required of exterior wall panels of buildings is that the exterior wall panels can resist deformation or falling-off when acted on by a strong wind due to a typhoon or the like. Past wind pressure resistance tests have shown that the part of an exterior wall board that is most likely to be damaged in the above wind pressure resistance 30 test is the part in which a locking piece on the overlying rabbeted portion side 4 of the board comes into contact with the fastening member. For this reason, when designing a fastening member, the design of the area of the locking portion, which is locked on the exterior wall board, of the fastening member is an important design consideration. In particular, in order to prevent 5 breakage of the upwardly extending portion of the locking portion and the locking piece on the overlying rabbeted portion side of the board, an increase in the area of contact between the upwardly extending portion of the locking portion and the locking piece on the overlying rabbeted portion side of the board is desirable, and this contact area has been a major factor in 10 determining the size of the fastening member. Thus, there has been a need to develop a fastening member having a locking portion that is capable of maintaining or improving the wind pressure resistance while having a still smaller shape than a conventional fastening member. 15 SUMMARY OF THE INVENTION The present invention has been made in order to address the above-described problems, and it is an object of the invention to provide an improved fastening member formed from a single steel sheet that can be manufactured using a smaller amount of steel than a conventional fastening 20 member and that therefore can be lightweight and cost-effective, and that is equipped with a locking portion having sufficient strength and is therefore capable of stably supporting an exterior wall board. A fastening member according to an aspect of the present invention is a fastening member for an exterior wall board. The fastening member 25 includes a left-side support portion and a right-side support portion on left and right sides thereof, the support portions being adapted to abut on a back surface of a board when the board is fastened. The fastening member further includes in an intermediate region between the left-side and right-side support portions, in sequence from the top: a base plate portion 30 that serves as a mount portion to be fixed to a building frame; a central 5 support portion that is adapted to abut on the back surface of a board when the board is fastened; a locking portion that extends from a lower edge of the central support portion and that is adapted to be locked on an upper edge and a lower edge of respective boards when the boards are fastened; and a lower 5 support portion that is adapted to abut on the back surface of a board when the board is fastened. The fastening member is formed from a single steel sheet. The left-side support portion, the right-side support portion, the lower support portion, and the central support portion are continuously formed by drawing the steel sheet. The base plate portion is formed by 10 depressing a portion of an upper region of the steel sheet toward a back surface thereof. The locking portion is formed by cutting a portion of a lower region of the steel sheet and raising that portion toward a front surface thereof. An end of the locking portion forms an upwardly extending portion, and a downwardly extending portion is formed by being cut and raised 15 toward a back surface of the base plate portion from a position at which the locking portion extends from the central support portion. In the above fastening member, the locking portion is provided by cutting out a portion of the steel sheet as the material for the fastening member and then utilizing the cut-out steel sheet. Thus, the fastening 20 member can be manufactured entirely from a single steel sheet, so that the weight and cost of the fastening member of the present invention can be reduced as compared with the conventional fastening member 50 shown in FIG. 5. Moreover, since the locking portion is extended from a support portion, the length of the locking portion can be reduced as compared with, 25 for example, the conventional fastening member 70 shown in FIG. 6 in which the locking portion 73 is extended from the base plate portion 71. As a result, the locking portion of the present invention has sufficient strength despite the fact that it is made from a single sheet. Furthermore, in the locking portion of the present invention, the locking portion is formed by 30 cutting a portion of the lower region of the steel sheet and raising that 6 portion toward the front surface, the end of the locking portion forms the upwardly extending portion, and the downwardly extending portion is formed by being cut and raised toward the back surface of the base plate portion from the position at which the locking portion extends from the central support 5 portion. In the locking portion of the fastening member of the present invention, the end of the extending portion is entirely used to form the upwardly extending portion. On the other hand, in the conventional fastening member shown in FIG. 7, the end of the extending portion is 10 divided to form the upwardly extending portions and the downwardly extending portion. Accordingly, the sum of the areas of the upwardly extending portions and the downwardly portion of the conventional fastening member shown in FIG. 7 is equal to the area of the upwardly extending portion of the fastening member of the present invention shown in FIG. 1. 15 Thus, the wind pressure resistance can be improved without increasing the external dimensions as compared with those of the conventional fastening member. Alternatively, in the case where the same wind pressure resistance is to be maintained, it is possible to design a smaller fastening member than the conventional fastening member. 20 Preferably, portions on left and right sides of the downwardly extending portion of the locking portion near the lower edge of the downwardly extending portion are bent toward the back surface of the fastening member such that the bent portions press against an exterior wall board when the board is fastened. In this embodiment, the board can be 25 maintained in such a position that it is continuously pressed against the support portions, thereby stabilizing the fastened state. Moreover, by forming the tips of the bent portions in such a shape that the tips cut into the surface of the board, additional fastening stability can be provided by a wedge effect. 30 Preferably, at least one support portion of the right-side support 7 portion, the left-side support portion, and the lower support portion includes a flange portion extending outward from an outer edge of that support portion, and the flange portion is coplanar with the base plate portion. In this embodiment, the flange portion as well as the base plate portion is 5 abutted against the building frame, so that the fastening member can be fixed to the building frame more stably. Preferably, the base plate portion is provided with cuts at its upper edge near the left-side support portion and the right-side support portion. In this embodiment, should a strong wind pressure (negative pressure) act on a 10 board that is fastened, areas of the base plate portion near the cuts are temporarily deformed to absorb the wind pressure (negative pressure), thereby preventing an excessive load from being put on the rabbeted portion of the board that is locked on the locking portion. Thus, a stable fastened condition can be maintained for a long period of time. 15 As described above, the fastening member of the present invention is entirely made from a single sheet and is therefore a lightweight and low-cost product, and yet the fastening member is capable of reliably supporting the board via the locking portion and support portions thereof. Moreover, since the end of the extending portion can be entirely used to form the upwardly 20 extending portion of the locking portion, the wind pressure resistance can be improved with a fastening member having the same size as a conventional fastening member. Alternatively, it is possible to design a fastening member having a smaller size than a conventional fastening member while maintaining the same wind pressure resistance. 25 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing a fastening member according to an embodiment of the present invention. FIG. 2 is a plan view showing a steel sheet as the material for the 30 fastening member shown in FIG. 1. 8 FIG. 3 is a perspective view showing a fastening member according to another embodiment of the present invention. FIG. 4 is a diagram showing an example of an exterior wall board. FIG. 5 is a diagram showing an example of a conventional fastening 5 member. FIG. 6 is a diagram showing another example of a conventional fastening member. FIG. 7 is a diagram showing another third example of a conventional fastening member. 10 FIG. 8 is a plan view showing a steel sheet as the material for the fastening member shown in FIG. 7. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a fastening member according to an embodiment of the 15 present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing the fastening member according to this embodiment of the present invention. A fastening member 10 as shown includes a left-side support portion 12L and a right-side support portion 12R on the left and right sides thereof 20 that are adapted to abut on a back surface of an exterior wall board when the board is fastened. The fastening member 10 also includes, in an intermediate region between the left-side and right-side support portions 12L and 12R, in sequence from the top: a base plate portion 11 that serves as a mount portion to be fixed to a building frame; a central support portion 12C 25 that is adapted to abut on the back surface of a board when the board is fastened; a locking portion 21 to be locked on the upper edge and the lower edge of respective boards when the boards are fastened; and a lower support portion 12U that is adapted to abut on the back surface of a board when the board is fastened. 30 The fastening member 10 is formed by punching, drawing and 9 bending a single steel sheet, such as a stainless steel sheet or an alloy plated steel sheet. FIG. 2 shows a steel sheet 100 as the material for the fastening member 10. The steel sheet 100 has a thickness of about 0.5 mm to 1.0 mm, which is rather thick, such that the number of reinforcing ribs to be provided 5 in the fastening member 10 can be reduced. Note that, in order to elucidate the correspondence between the parts of the steel sheet 100 and the parts of the finished fastening member 10, the numerals in parentheses shown in FIG. 2 indicate the corresponding parts of the fastening member 10 of FIG. 1. An example of the process of manufacturing the fastening member 10 10 will be described below. First, the steel sheet 100 (see FIG. 2), in which fixing screw openings 31, nail openings 34, cuts 36, and a cut N are already formed, is punched. The cut N is formed in a lower region 100B of the steel sheet 100 substantially in a U-shape as seen in a plan view. The area surrounded by the cut N forms an extending portion 102 that serves as the 15 locking portion 21 of the fastening member 10 when the processing of the steel sheet is finished. The extending portion 102 includes, at the end thereof, one projecting portion 102a that serves as an upwardly extending portion 22 of the locking portion 21 after processing. A cut M is formed in the lower region 100B of the steel sheet 100 20 substantially in an inverted U-shape as seen in a plan view, and a downwardly extending portion 23 is formed by being cut and raised toward the back surface of the base plate portion from a position at which the locking portion 21 extends from the central support portion 12C. The area surrounded by the cut M includes one projecting portion 102b that serves as 25 the downwardly extending portion 23 of the locking portion 21 after processing. The steel sheet 100 is then drawn along the two-dot chain lines of FIG. 2 such that the steel sheet 100 partly protrudes to form the left-side support portion 12L and the right-side support portion 12R on either side of 30 the steel sheet 100 and the central support portion 12C and the lower support 10 portion 12U in the lower region 100B of the steel sheet 100. As a result, in a central portion 101 of the upper region 100A of the steel sheet 100, the base plate portion 11 that is depressed toward the back surface by about several millimeters is formed. Furthermore, a flange portion 13 is formed along 5 outer edge portions of the left-side, right-side, and lower support portions 12L, 12R, and 12U. During the aforementioned drawing process, a raised portion 32 within which nail openings 34 are to be disposed and reinforcing ribs 35 are simultaneously formed. Then, the downwardly extending portion 23 formed by the cut M that 10 is formed in the lower region 100B of the steel sheet 100 in advance is bent obliquely downward along the lower side thereof (dashed line L2b in FIG. 2) toward the back surface. Moreover, the extending portion 102 that is formed in the lower region 100B of the steel sheet 100 in advance is bent at a right angle along the upper side thereof (dashed line Li in FIG. 2) toward the front 15 surface. Furthermore, the projecting portion 102a at the end of the extending portion 102 is bent obliquely upward along dashed line L2a of FIG. 2 such that the upwardly extending portion 22 is formed. The locking portion 21 is thus formed in a shape that conforms to the rabbeted portions at the upper edge and the lower edge of the respective boards that are fastened. 20 Furthermore, edge portions on the left and right sides of the end of the downwardly extending portion 23 of the locking portion 21 are bent toward the back surface along dashed lines L3 of FIG. 2 such that bent portions 23a are formed. While the fastening member 10 is completed by the above-described 25 bending process, the fastening member of the present invention is not limited to the above-described embodiment. For example, the flange portion 13 and the cuts 36 may be omitted. FIG. 3 shows another embodiment of the present invention. The locking portion 21 shown in FIG. 3 includes, at an end portion thereof, one 30 upwardly extending portion 22 and two downwardly extending portions 23. 11 In this embodiment, since the downwardly extending portions 23 are formed on either side, the punching process of the cut M and the bending process in which the downwardly extending portions 23 are bent toward the back surface of the base plate portion are facilitated. 5 12