A zoom lens barrel capable of decreasing a distance between a lens and a diaphragm in an optical axis direction to thereby increase the photographing magnification and at the same time to reduce the lens barrel size, and capable of changing a diaphragm aperture diameter according to zoom control. The lens barrel has a lens relatively movable toward and away from an iris diaphragm in the optical axis direction with a change in the photographing magnification. An operation of the iris diaphragm is controlled in such a manner that a minimum diameter of an aperture of diaphragm blades of the iris diaphragm changes according to an amount of intrusion of a part of the lens into the aperture of the diaphragm blades in a range where the diaphragm blades do not interfere with the lens.