An optical stop unit includes plural light blocking plates, and a cam
mechanism for driving the plural light blocking plates to vary an
aperture diameter, the cam mechanism including a cam follower and a cam
groove, wherein the cam follower and cam groove have mutually facing
surfaces, through which the cam follower and cam groove contact each
other, and at least one of which surfaces has a curved section that is
substantially perpendicular to a plane formed by the plural light
blocking plates.