Abstract: In a ball screw 1 in which a screw shaft 2 having a spiral-shaped screw shaft side ball rolling groove 6 in an outer periphery is screwed into a nut 4 having in an inner periphery a nut side ball rolling groove 8 facing the screw shaft side ball rolling groove 6, a plurality of load balls 10 is rollably disposed in a spiral-shaped load rolling path 12 formed by the screw shaft side ball rolling groove 6 and the nut side ball rolling groove 8, spacer balls 20 having a smaller diameter than load balls 10 are evenly disposed with respect to strings of the load balls 10, and a ratio of a number of spacer balls 20 to a number of load balls 10 (a spacer ball ratio) is set to be in a range of 1% to 5%.