A zoom lens system includes four lens units each having positive, negative, negative and positive refractive powers in the stated order from an object side to an image side. A first lens unit includes a variable apex angle prism on the image side of at least one lens element. In this zoom lens system, a focal length of the first lens unit and imaging magnifications of a second lens unit and a fourth lens unit are set appropriately so as to suppress occurrence of decentering aberration accompanying a variation of a prism apex angle.