A zoom lens barrel comprises a holding drum for holding a stop mechanism and a lens unit, a driving mechanism for making the holding drum move back and forth in an optical-axis direction to adjust a focal length of an optical system, and a stop compensating mechanism for compensating an aperture diameter of the stop mechanism by rotating the holding drum about the optical axis, corresponding to the movement of the holding drum in the optical-axis direction. The holding drum has a rotation permitting portion for enabling a portion for holding the lens unit make a rotational motion about the optical axis with respect to a portion for holding the stop mechanism.