To provide a ball screw allowing a fixing member for a bearing or the like to be installed without looseness in an axial direction by using an inexpensive, widely available retaining ring.

This ball screw is provide for regulating the movement of the fixing member 16 of the bearing 15 by the retaining ring 20 installed in an installation groove 30 arranged in a nut 12, and is characterized in that both side surfaces of the retaining ring 20 installed in the installation groove 30 are formed in a parallel shape, and a groove side surface 31 positioned on the anti-fixing member side of the installation groove 30 engaging with an inner diameter end part of the retaining ring 20 is formed in a tapered shape, and a contact height h2 from the groove bottom of a contact part 22a between the retaining ring 20 and the fixing member 16, becomes higher than a contact height h1 from the groove bottom of a contact part 21a between the retaining ring 20 and the groove side surface 31 of a tapered shape.