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, all lens units moves in a direction along the optical axis such that intervals among respective lens units varies. Further, at the time of zooming, the third lens unit G3 moves in the direction along the optical axis such that the interval between the second lens unit G2 and the third lens unit G3 is made longer at a telephoto limit than the interval at a wide-angle limit. Still further, condition (6): −1.6