A ball screw includes a screw shaft, in which a thread groove is formed in an outer peripheral face thereof, that includes a small-diameter portion with a diameter smaller than an outer diameter of the screw shaft formed integrally at one end of the screw shaft; a nut, through which the screw shaft passes, having a thread groove formed in an inner peripheral face thereof; a plurality of balls disposed in a ball passage formed by the thread groove of the screw shaft and the thread groove of the nut; and a ring-shaped stopper, immovably fitted on the small-diameter portion, that prevents the nut from moving.