Provided is a zoom lens system including a compact focusing lens unit and having a suppressed change in image magnification at the time of movement of the focusing lens unit. The zoom lens system of the present invention, in order from an object side to an image side, includes a first lens unit G1 having positive optical power, a second lens unit G2 having negative optical power, a third lens unit G3 having negative optical power, and a fourth lens unit G4 having positive optical power. At the time of zooming, at least the first lens unit G1 moves from a wide-angle limit to a telephoto limit. The fourth lens unit G4 includes a first sub lens unit having positive optical power and a second sub lens unit having negative optical power, the second sub lens unit being arranged at the image side relative to the first sub lens unit. At the time of compensating image blur caused by vibration applied to the zoom lens system, the first sub lens unit or the second sub lens unit moves in a direction perpendicular to the optical axis.