A mechanism which enables reduction of the diametrical size of a lens barrel of an optical apparatus. In the lens barrel, a third group lens unit has a rotation restricted portion. A fourth group lens holder rearward of the third group lens unit has a rotation restricting portion. A sixth group lens holder rearward of the fourth group lens holder has a rotation restricting portion. The rotation restricted portion of the third group lens unit and the rotation restricting portion of the fourth group lens holder are mutually engaged, whereby relative rotation between the lens unit and the lens holder is restricted, and the rotation restricted portion of the third group lens unit and the rotation restricting portion of the sixth group lens holder are mutually engaged, whereby relative rotation between the lens unit and the lens holder is restricted.