A zoom lens system comprising a first lens unit of negative power, a second lens unit of positive power and a third lens unit of positive power, wherein in zooming, the lens units move respectively along an optical axis so that an interval of first lens unit and second lens unit decreases while an interval of second lens unit and third lens unit changes with variable magnification, the first lens unit comprises one object side negative lens element and one image side positive lens element, which have an aspheric surface, and the conditions: n12>1. 88, v12<26 and 5.0<.alpha.i.sub.W<20.0 (.omega..sub.W>30, n12 and v12 are refractive index and Abbe number, respectively, of the image side positive lens element of the first lens unit, .alpha.i .sub.W is an incident angle of the principal ray onto the image sensor at the maximum image height at a wide-angle limit) are satisfied.