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 comprises: a positive lens unit that is arranged on an object side relative to the aperture diaphragm; a negative lens unit that is arranged on an image side relative to the positive lens unit and on an object side relative to the aperture diaphragm; and a focusing lens unit that is arranged in an optical path between the negative lens unit and the aperture diaphragm. The zoom lens system satisfies the condition (4): 1.20 3.0, βNT(βNW): a composite focal length of the focusing lens unit and the negative lens unit in an infinity in-focus condition at a telephoto (wide-angle) limit when the focusing lens unit has negative optical power, or a lateral magnification of the negative lens unit at a telephoto limit in an infinity in-focus condition when the focusing lens unit has positive optical power, fT: a focal length of the entire system at a telephoto limit, fW: a focal length of the entire system at a wide-angle limit.