To provide a ball screw capable of correctly aligning balls moving from a noload return passage of a circulation member to a load ball rolling groove of a nut.

A section area shape of the load ball rolling groove 2a of the nut 2 is formed as a Gothic arch groove form including two circular curves having curvature radius longer than a radii of the balls 3. A section area shape of a part of the nut 2 connected to the load ball rolling groove 2a at an end part of the noload return passage 14 of the circulation member 8 in a longitudinal direction is formed as the Gothic circular curve in accordance with the section area shape of the load ball rolling groove 2a of the nut 2.