A cam mechanism used is provided with: a cam ring forming a circle around an axis and including a plurality of cam faces arranged circumferentially on a face of the circle facing in an axial direction, each of the cam faces sloping in a circumferential direction relative to a circumferential face perpendicular to the axis; a pressure ring adjacent axially to the face of the cam ring and including a plurality of cam faces opposed to the face of the cam ring and respectively symmetrical to the plurality of cam faces of the cam ring, the pressure ring being rotatable relatively to the cam ring about the axis; and a plurality of taper rollers interposed between the cam ring and the pressure ring, each of the taper rollers including a conical face tapering radially inwardly and capable of rolling on the cam faces.