An assembly apparatus for assembling a ball screw device which comprises: a screw shaft provided with a spiral shaft raceway groove on an outer circumference thereof; a cylindrical nut provided with a nut raceway groove on an inner circumferential face thereof so as to oppose to the shaft raceway groove; a connection passage that makes a circulation passage together with the shaft raceway groove and the nut raceway groove; and a plurality of balls circulating in the circulation passage, the assembly apparatus comprising: a support block; a nut arranged in the longitudinal direction; a nut reception base that arranges the nut so that its axial direction is towards a vertical direction of the assembly apparatus and that supports the nut so as to elevate the nut with respect to the support block; a temporary shaft that is inserted into an inner diameter side of the nut arranged on the nut reception base and that is rotatably supported by the support block; an inclined face provided at a forward end portion of the temporary shaft, for guiding balls, the number of which is the same as that of the balls to be charged into the circulation passage into the nut raceway groove; and a pushing mechanism that has elasticity in the radial direction of the temporary shaft and that pushes balls which are fed into the nut raceway groove by a forward end portion protruding from the outer circumferential face of the temporary shaft, characterized in that the pushing mechanism comprises: a protruding member protruding from the outer circumferential face of the temporary shaft; and an annular elastic member arranged on the central side of the temporary shaft for pushing the protruding member. a screw shaft provided with a spiral shaft raceway groove on an outer circumference thereof; a cylindrical nut provided with a nut raceway groove on an inner circumferential face thereof so as to oppose to the shaft raceway groove; a connection passage that makes a circulation passage together with the shaft raceway groove and the nut raceway groove; and a plurality of balls circulating in the circulation passage, the assembly apparatus comprising: a support block; a nut arranged in the longitudinal direction; a nut reception base that arranges the nut so that its axial direction is towards a vertical direction of the assembly apparatus and that supports the nut so as to elevate the nut with respect to the support block; a temporary shaft that is inserted into an inner diameter side of the nut arranged on the nut reception base and that is rotatably supported by the support block; an inclined face provided at a forward end portion of the temporary shaft, for guiding balls, the number of which is the same as that of the balls to be charged into the circulation passage into the nut raceway groove; and a pushing mechanism that has elasticity in the radial direction of the temporary shaft and that pushes balls which are fed into the nut raceway groove by a forward end portion protruding from the outer circumferential face of the temporary shaft, a support block; a nut arranged in the longitudinal direction; a nut reception base that arranges the nut so that its axial direction is towards a vertical direction of the assembly apparatus and that supports the nut so as to elevate the nut with respect to the support block; a temporary shaft that is inserted into an inner diameter side of the nut arranged on the nut reception base and that is rotatably supported by the support block; an inclined face provided at a forward end portion of the temporary shaft, for guiding balls, the number of which is the same as that of the balls to be charged into the circulation passage into the nut raceway groove; and a pushing mechanism that has elasticity in the radial direction of the temporary shaft and that pushes balls which are fed into the nut raceway groove by a forward end portion protruding from the outer circumferential face of the temporary shaft, characterized in that the pushing mechanism comprises: a protruding member protruding from the outer circumferential face of the temporary shaft; and an annular elastic member arranged on the central side of the temporary shaft for pushing the protruding member. a protruding member protruding from the outer circumferential face of the temporary shaft; and an annular elastic member arranged on the central side of the temporary shaft for pushing the protruding member.