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; a third lens unit having negative optical power; and at least one subsequent lens unit, wherein at the time of zooming, all lens units move in a direction along the optical axis so that intervals between the lens units vary, and at the time of focusing, a lens unit included in the at least one lens unit moves in a direction along the optical axis.