A zoom lens system is disclosed, comprising, in order from an object side to an image side, a first lens unit with a positive refractive power, a second lens unit with a negative refractive power, a third lens unit with a positive refractive power, and a fourth lens unit with a positive refractive power, wherein a distance between the respective lens units is changed during zooming. The third lens unit consists of, in order from the object side to the image side, a first lens sub-unit including a positive lens with an object-side surface which has a convex shape and having a positive refractive power, an aperture stop, a second lens sub-unit including a negative lens and having a negative optical power, and a third lens sub-unit including a positive lens and having a positive optical power.