A zoom lens system is disclosed, comprising, in order from an object side to an image side, a first lens unit of positive optical power, a second lens unit of negative optical power, a third lens unit of positive optical power, and a fourth lens unit of positive optical power, wherein the second and fourth lens units are moved during zooming. The third lens unit consists of a first lens subunit of positive optical power, an aperture stop, and a second lens subunit having one or more negative lens elements and one or more positive lens elements, in order from the object to the image sides. Optical parameters including the focal length of each lens unit, lens shapes, the lens structure of each lens unit are specified to realize a zoom lens system with a smaller size of the entire lens system and higher optical performance.