A two-unit zoom lens system which comprises a negative first lens unit G1 and a positive second lens unit G2, during change of magnification from a wide-angle end to a telephoto end, the first lens unit and the second lens unit are moved so as to reduce a space between the first lens unit and the second lens unit, the first lens unit includes a negative sub-unit G1a and a positive sub-unit G1b, the second lens unit has a first positive lens L21, a second negative lens L22, and a third positive lens L23, and the system further has an aperture stop S disposed in an air space which comes into contact with the first positive lens L21.