A zoom lens system for forming an optical image of a subject on the image-sensing surface of an image sensor includes at least, from the object side thereof, a first lens unit having a negative optical power and a second lens unit having a positive optical power, and achieves zooming by varying the distances between the individual lens units in such a way that, during zooming from the wide-angle end to the telephoto end, the distance between the first and second lens units decreases. The first lens unit consists of two plastic lens elements.