A piston of the present invention is pressed toward an engagement position by the hydraulic pressure of a hydraulic chamber. A first return spring is in contact with the piston from the engagement position to a standby position, and urges the piston toward a release position. The standby position is between the release position and the engagement position. A second return spring is in contact with the piston from the engagement position to the release position, and urges the piston toward the release position. The center axis of the first return spring is arranged to match the center axis of a rotating shaft. The center axis of the second return spring is arranged to match the center axis of the rotating shaft.