A zoom lens system comprising, in order from the object side, a first positive lens unit, a second lens unit which is moved monotonously during zooming mainly for sharing a vari-focal function and has a positive power, a third positive lens unit, and a fourth lens unit which is moved during zooming forward and backward mainly for correcting a location of an image surface and has a positive power; the third lens unit consisting of a first lens component which comprises at least one positive lens element and a second lens component which comprises at least one negative lens element, whereas the fourth lens unit consisting of a first lens component which comprises at least one negative lens element and a second lens component which comprises at least one positive lens element.