To provide a ball screw smoothing the movement of balls on all of portions where the balls move.

The ball screw includes a threaded shaft 1 having on its outer peripheral surface a helical thread groove 1a; a nut 2 having on its inner peripheral surface a thread groove 2a opposite to the thread groove 1a of the threaded shaft 1 and engaged with the threaded shaft 1 via a number of balls 3 rollably loaded into a helical ball rolling path 5 formed by both thread grooves 1a and 2a; and a return tube 6 secured to the nut 2 for scooping up the balls 3 at one end A of the ball rolling path 5 to feed the balls to the other end. At a portion E1 near the one end A of the ball rolling path 5, the lead angle or the diameter of the thread groove 2a is gradually varied so that preloads decrease toward the one end A of the ball rolling path 5. The preloads are eliminated at a portion B in front of the one end A of the ball rolling path 5. Both ends 6b of the return tube 6 are attached to the nut 2 along the direction of the lead angle and the tangential direction of the ball rolling path 5.