A zoom lens barrel includes first and second lens groups arranged along an optical axis. A first drive mechanism moves the first lens group only in the direction of the optical axis for focusing. A second drive mechanism moves both the first and second lens groups in the direction of the optical axis for zooming. A rotary barrel which is rotated by the second drive mechanism is provided with a cam groove which guides the second lens group in response to rotation of the rotary lens barrel during zooming. A cam follower is provided on the lens holder of the second lens group and is in engagement with the cam groove. The cam groove consists of alternately formed zooming groove portions and flange back adjustment groove portions, and flange back adjustment is carried out by rotating the rotary barrel with the cam follower in engagement with one of the flange back adjustment groove portions.