A cam barrel rotating mechanism of a zoom lens includes a plurality of lens groups guided in a direction of an optical axis without rotating about the optical axis to change a focal length of the zoom lens; a cam barrel rotatably supported to move the plurality of lens groups in the optical axis direction; a rotational barrel positioned around the cam barrel, the rotational barrel being rotatably driven; a plurality of rotation transmission grooves formed on an outer peripheral surface of the cam barrel, at an equi-angular intervals, extending parallel to the optical axis; and a plurality of inward projections formed on an inner peripheral surface of the rotational barrel to be engaged in the plurality of rotation transmission grooves, respectively.