The invention provides a zoom lens system much more reduced in size and cost than ever before and best-suited for use on portable information terminals of small size. The zoom lens system comprises, in order from an object side thereof, a first lens group G1 having positive refracting power and designed to be fixed during zooming, a second lens group G2 having negative refracting power and designed to move from the object side to an image plane side of the system for zooming from a wide-angle end to a telephoto end of the system, a third lens group having refracting power and designed to move from the image plane side to the object side for zooming from the wide-angle end to the telephoto end, and a fourth lens group G4 having positive refracting power and designed to be movable during zooming. Condition (1) with respect to the power of the third lens group G3, condition (2) with respect to the amount of zooming movement of the third lens group G3 or condition (3) with respect to the composite power of the third and fourth lens groups G3 and G4 and condition (10) with respect to the actual value of the back focus are satisfied.