A housing has a screw shaft passing wall in which a through hole, through which a screw shaft passes, is formed. A spline outer cylinder is held at one end by the screw shaft passing wall, and a disc spring that biases the spline outer cylinder toward the screw shaft passing wall is arranged at the other end of the spline outer cylinder. A tapered recess is formed in one end face of the spline outer cylinder, and a tapered projection that is fitted in the tapered recess is formed in a face of the screw shaft passing wall of the housing, which faces the spline outer cylinder.