A zoom lens system for forming an optical image of a subject on the light-receiving surface of an image sensor that converts the optical image into an electrical signal is provided with, from the object side thereof, at least a first lens unit, a second lens unit, and a third lens unit, and achieves zooming by varying the distances between the lens units. The first lens unit has a negative optical power. The second lens unit has a positive optical power. A prescribed condition is fulfilled.