A zoom lens system is disclosed, in which the decrease of the peripheral light quantity at the wide-angle end is improved. The zoom lens system comprises, in order from an object side to an image side, a first lens unit having a positive optical power, a second lens unit having a negative optical power, an aperture stop, a third lens unit having a positive optical power and a fourth lens unit having a positive optical power. The four lens units and the aperture stop are moved during zooming. When MS represents movement amount of the aperture stop during zooming from the wide-angle end to the telephoto end and M3 represents the maximum movement amount of the third lens unit in the optical axis direction during zooming from the wide-angle end to the telephoto end, −1.8