PURPOSE: To keep constant the contact angle of a cam follower and achieve a stable clutching/declutching performance by forming a cam face on a curvature which is curving in the same direction as the circular internal circumferential surface, wherein the cam face forms a gap narrowing wedge-shapedly in the circumferential direction between it and the circular internal circumferential surface.

CONSTITUTION: Mating with the circular internal circumferential surface of a driving side member 6, cam faces 5c, 5d forming a gap narrowing wedge- shapedly in the circumferential direction with respect to the mentioned circular internal circumferential surface 60a are provided on a following side member. These cam faces 5c, 5d are embodied as curvature curving in the same direction as this circular internal circumferential surface 60a. The contact angle of cam followers 7a, 7b is kept approx. constant despite varying position in which the cam followers 7a, 7b are bitten in between the circular internal circumferential surface 60a and the cam face 5c on the following side member. Thus a stable clutching/declutching performance is achieved.