A drive bush configured to circumferentially rotate and drive a pair of wedge members in an eccentric space includes a cylindrical portion rotatably supported inside a boss portion of an internal tooth constituting member, a flange portion integrally formed at a first end portion in the axial direction of the cylindrical portion so as to face one side surface side of the wedge members, and a pushing portion integrally provided on the outer peripheral side of the flange portion, and configured to release a wedge effect by circumferentially pushing the wedge members. The inner surface of the cylindrical portion is integrally formed with a wall portion, and the wall portion is formed with, in the middle thereof, a female spline teeth meshing with the output shaft of a motor. The cylindrical portion, the wall portion including the female spline teeth and the flange portion have substantially the same thickness.