A zoom lens system including, in order from an object side to an image side, a first lens unit of positive optical power, and a second lens unit of negative optical power, followed by a lens component having a lens unit, wherein at least one of the first lens unit and the second lens unit moves during zooming. One of the lens units of the zoom lens system includes a layer made of a material that satisfies the following conditions: 11<.nu.IT<27 0.2<.theta.IT<0.4. Wherein .nu.IT is an Abbe number of the layer, and .theta.IT is a partial dispersion ratio for the g-spectral line and f-spectral line of the layer. The layer has an optical power of a sign opposite to that of the lens unit including the layer.