Abstract: A clutch comprising a drive rotary member, a driven rotary member engageable with and disengageable from the drive rotary member by being moved relative thereto in the directions of axis of rotation, an annular cam interposed between the drive rotary member and the driven rotary member, and an engaging member manually operably mounted on a case to prevent rotation of the cam. When the engaging member is operated to project toward the driven rotary member through the cam, the driven rotary member is held out of engagement with the drive rotary member reliably.