Ball screw

An object of the present invention is to improve the durability of a ball screw under the circumstances in which the radial load and the moment load are applied to it. Ball screw comprising a screw shaft (2) formed on its outer circumferential surface with a helical screw groove (2a), a nut (3) adapted to mate with the screw shaft (2) and formed on its inner circumferential surface with a helical screw groove (3a) corresponding to the screw groove (2a) of the screw shaft (2), a number of balls (4) rollably contained within a ball rolling passage formed by the oppositely arranged screw grooves (2a,3a), and bridge members (5) each formed on its inner circumferential surface with a connecting groove (5a) for connecting the screw grooves (3a) of the nut (3) wherein the contact angle (±) between the ball (4) and the screw groove (2a,3a) is set within a range of 20° to 40°.