A zoom lens system that performs zooming by moving lens units includes, in the following order from an object side towards image side, a first lens unit having positive refractive power and movable along a locus convex towards the image side during zooming from a wide-angle end to telephoto end, a second lens unit having negative refractive power, an aperture stop movable independently of the lens units, and third and fourth lens units having positive refractive powers. The following conditions are satisfied: −0.6