There is disclosed a zoom lens system comprising, in order from an object side: a first lens unit having a positive refractive power; a second lens unit having a negative refractive power; a third lens unit having a positive refractive power; and a fourth lens unit having a negative refractive power. During zooming from a wide-angle end to a telephoto end, a space between the first lens unit and the second lens unit enlarges, a space between the second lens unit and the third lens unit narrows, a space between the third lens unit and the fourth lens unit changes, and all of the first to fourth lens units move along an optical axis. The zoom lens system further comprises an aperture stop which moves integrally with the third lens unit during the zooming.