A ball screw device, comprising: a plurality of balls; a screw shaft having a spiral rolling groove in an outer peripheral surface thereof, the spiral rolling groove being configured to allow the plurality of balls to roll therein; and a nut member which is threadedly engaged with the screw shaft through intermediation of the plurality of balls, wherein the nut member includes a plurality of nut components (5, 6) each having a through hole configured to allow the screw shaft to be inserted therethrough, the plurality of nut components (5, 6) being arranged in a superposed manner along an axial direction of the screw shaft, wherein, at a boundary portion between the plurality of nut components (5, 6) which are adjacent to each other along the axial direction of the screw shaft, a raceway path extending around a periphery of the screw shaft is formed as an endless circulation path for the plurality of balls, wherein the plurality of nut components (5, 6) each have a joining surface used for joining with another one of the plurality of nut components (5, 6), the joining surfaces each including: a raceway forming surface (53, 62) which is annularly formed around the through hole; and an abutment surface (50, 60) for joining which is formed, on an outer side in a radial direction with respect to the raceway forming surface (53, 62), wherein the raceway forming surface (53, 62) has a curved-surface shape along the raceway path, and the abutment surface (50, 60) for joining is formed as a flat surface perpendicular to the axial direction of the screw shaft, and wherein the raceway forming surfaces (53, 62) formed on the plurality of nut components (5, 6) have the same shape, wherein the plurality of nut components (5, 6) each have a ball circulation groove (55, 64), which is formed at a corner portion at which the raceway forming surface (53, 62) and an inner peripheral surface of the through hole intersect each other, wherein the ball circulation groove (55, 64) includes: a load groove portion (55a, 64a) configured to allow the plurality of balls to roll between the screw shaft and the load groove portion (55a, 64a) while bearing a load; and a no-load groove portion (55b, 64b) connecting both ends of the load groove portion (55a, 64a), and wherein the raceway forming surfaces (53, 62) of the plurality of nut components (5, 6) which are adjacent to each other are superposed on each another to allow the ball circulation grooves (55, 64) to form the raceway path, characterized in that the raceway forming surface (53, 62) has such a shape that a half of the raceway forming surface (53, 62) that surrounds the periphery of the screw shaft projects with respect to the abutment surface (50, 60) for joining, and another half thereof is recessed with respect to the abutment surface (50, 60) for joining. a plurality of balls; a screw shaft having a spiral rolling groove in an outer peripheral surface thereof, the spiral rolling groove being configured to allow the plurality of balls to roll therein; and a nut member which is threadedly engaged with the screw shaft through intermediation of the plurality of balls, wherein the nut member includes a plurality of nut components (5, 6) each having a through hole configured to allow the screw shaft to be inserted therethrough, the plurality of nut components (5, 6) being arranged in a superposed manner along an axial direction of the screw shaft, wherein, at a boundary portion between the plurality of nut components (5, 6) which are adjacent to each other along the axial direction of the screw shaft, a raceway path extending around a periphery of the screw shaft is formed as an endless circulation path for the plurality of balls, wherein the plurality of nut components (5, 6) each have a joining surface used for joining with another one of the plurality of nut components (5, 6), the joining surfaces each including: a raceway forming surface (53, 62) which is annularly formed around the through hole; and an abutment surface (50, 60) for joining which is formed, on an outer side in a radial direction with respect to the raceway forming surface (53, 62), wherein the raceway forming surface (53, 62) has a curved-surface shape along the raceway path, and the abutment surface (50, 60) for joining is formed as a flat surface perpendicular to the axial direction of the screw shaft, and wherein the raceway forming surfaces (53, 62) formed on the plurality of nut components (5, 6) have the same shape, wherein the plurality of nut components (5, 6) each have a ball circulation groove (55, 64), which is formed at a corner portion at which the raceway forming surface (53, 62) and an inner peripheral surface of the through hole intersect each other, wherein the ball circulation groove (55, 64) includes: a load groove portion (55a, 64a) configured to allow the plurality of balls to roll between the screw shaft and the load groove portion (55a, 64a) while bearing a load; and a no-load groove portion (55b, 64b) connecting both ends of the load groove portion (55a, 64a), and wherein the raceway forming surfaces (53, 62) of the plurality of nut components (5, 6) which are adjacent to each other are superposed on each another to allow the ball circulation grooves (55, 64) to form the raceway path, characterized in that the raceway forming surface (53, 62) has such a shape that a half of the raceway forming surface (53, 62) that surrounds the periphery of the screw shaft projects with respect to the abutment surface (50, 60) for joining, and another half thereof is recessed with respect to the abutment surface (50, 60) for joining.