A cam mechanism includes a cam ring; and a linearly movable frame movable by the cam ring along an optical axis of an optical system, by engagement of a plurality of cam grooves located on the cam ring with a plurality of complementing cam followers located on the linearly movable frame when the cam ring is rotated, the linearly movable frame supporting at least one optical element of the optical system. The plurality of cam grooves are located at different positions in both the optical axis direction and a circumferential direction of the cam ring, and respectively trace substantially a same reference cam diagram. The plurality of complementing cam followers are located at different positions in both the optical axis direction and a circumferential direction of the linearly movable frame, and are engageable in the plurality of cam grooves, respectively.