A cam locus of the cam surface on the image plane side of the first group cam groove of the rotating cylinder has an angle to a flat surface where the optical axis crosses perpendicularly. When the cam follower engages with the cam surface on the image plane side, an impact force is dispersed into the optical axis direction and the rotating direction of the rotating cylinder, and a load on the cam follower and male helicoid is reduced. When the first group lens system is pressed by a light load, the cam follower of the first group lens movement frame engages with the cam surface on the image plane side, and an angle of the cam surface is equal to an angle of a force occurring in the rotating direction. Therefore, the rotating cylinder does not rotate easily, and returns to its previous position when the load is removed.