A zoom lens system is provided that includes a compactly constructed focusing lens unit and that has a suppressed change in the image magnification at the time of movement of a focusing lens unit. The zoom lens system according to the present invention, in order from an object side to an image side, comprises: a first lens unit having positive optical power; a second lens unit having negative optical power; and at least two subsequent lens units, wherein the two lens units includes a lens unit A having positive optical power and a lens unit B arranged on the image side relative to the lens unit A, at the time of zooming, all lens units move in a direction along the optical axis so that intervals between the lens units vary, at the time of focusing, the lens unit B moves in a direction along the optical axis, and at the time of compensating image blur caused by vibration in the zoom lens system, either the entirety of a lens unit arranged on the object side relative to the lens unit A or a sub lens unit which is a part of a lens unit arranged on the object side relative to the lens unit A and is arranged on the image side therein moves in a direction perpendicular to the optical axis.