A first spacer, a second tubular member, and a second spacer are externally fitted and fixed to an outer peripheral surface of a nut. The second tubular member is formed by stacking a plurality of silicon steel plates without any gap, and is arranged between the first spacer and the second spacer axially. The outer diameter of an outer peripheral surface of the second tubular member is made larger than the outer diameter of an outer peripheral surface of the first spacer and the outer diameter of an outer peripheral surface of the second spacer. A detection surface of a first displacement detector is made to radially face a contact portion between the second spacer and the second tubular member, while a detection surface of a second displacement detector is made to radially face a contact portion between the first spacer and the second tubular member.