A drive mechanism for a zoom lens system consisting of first and second lens groups has a movable lens barrel fixedly holding the first lens and cam structure consisting of a cam member provided on the second lens group and a cam follower member on the movable lens barrel. The movable lens barrel moves forward or rearward to shift the second lens group forward or rearward relative to the first lens group along the optical axis of the zoom lens system via the cam member so as to continuously change the focal length of the zoom lens system. The cam member is mounted on the movable lens barrel to change the curve of the cam surface relative to the cam follower member only in a middle focal length range.