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 plurality of lens units and an aperture diaphragm arranged in the lens unit. The plurality of lens units include: a negative lens unit that is arranged on an object side relative to the aperture diaphragm and provided with negative optical power having an absolute value greatest in the entire system and that moves in a direction along an optical axis at the time of zooming; and a focusing lens unit that is arranged in an optical path between the negative lens unit and the aperture diaphragm and that moves in a direction along the optical axis at the time of focusing such that an interval relative to the negative lens unit should vary.