A compact zoom lens comprising, from front to rear, a first lens unit of positive power for focusing, a second lens unit of negative power axially movable in one direction when zooming from one end of a range of image magnification to the other, a third lens unit of positive power axially movable in a path convex toward the rear to maintain the constant position of an image surface which shifts with zooming, and a fourth lens unit of positive power for forming an image which remains stationary during zooming, wherein a fixed diaphragm is arranged in a space between the second and third lens units.