A ball screw having a groove-shaped ball return passage has improved durability, ball circulation performance, and working performance. The ball screw is provided with: a threaded shaft having a spiral groove at its outer circumferential surface; a nut having a spiral groove which faces the spiral groove at its inner circumferential surface; a plurality of balls rotatably loaded in a spiral raceway constituted by the two spiral grooves; and a ball circulation groove which returns and circulates the balls from an end point to a start point of the raceway. Further, the edge portions constituted by the two side surfaces of the ball circulation groove and the surfaces which continue from the side surfaces and extend in the axial direction are formed to be rounded.