A zoom lens system includes a compact focusing lens unit and has a suppressed change in image magnification at the time of movement of the focusing lens unit. The system includes an aperture diaphragm, a first lens unit having negative optical power and being closest to the object side, a third lens unit arranged on the object side relative to the aperture diaphragm and being adjacent to the diaphragm, and a fourth lens unit arranged on the image side relative to the aperture diaphragm and being adjacent to the diaphragm. At the time of zooming, interval between the aperture diaphragm and the third lens unit arranged on the object side relative to the diaphragm and being adjacent to the diaphragm varies, and interval between the aperture diaphragm and the fourth lens unit arranged on the image side relative to the diaphragm and being adjacent to the diaphragm varies.