To obtain an advance/retreat cam mechanism for a lens barrel where the miniaturization of a cam ring is made compatible with the securement of the moving amount of a movable member.

In the advance/retreat cam mechanism for the lens barrel equipped with a movable member supporting an optical element and guided to advance straight in an optical axis direction, and a cam ring having a cam groove which a cam follower arranged on the movable member is engaged with on its peripheral surface, either front or rear end of the cam groove is opened to the end face of the cam ring, and the cam follower is moved to the open end of the cam groove or the outside position of the cam groove where it comes off from the open end in accordance with the rotation of the cam ring in one direction. The movable member and the cam ring are provided with a cam follower advance assistant part making the cam follower advance to the inside of the cam groove in accordance with the rotation in the reverse direction of the cam ring when the cam follower is positioned at the open end of the cam groove or it comes off the open end of the cam groove.