A zoom lens system has four lens units, i.e., in order from an object side to an image side, a first lens unit of negative optical power, a second lens unit of positive optical power, a third lens unit of positive optical power, and a fourth lens unit of positive optical power. During zooming from a wide-angle end to a telephoto end, a space between the first lens unit and the second lens unit decreases, a space between the second lens unit and the third lens unit varies, and a space between the third lens unit and the fourth lens unit increases. Two negative lens elements constituting the first lens unit are made from high refractive index materials. Accordingly, a small-sized zoom lens system capable of obtaining a desired wide angle of field at the wide-angle end, a desired zoom ratio and high optical performance with a minimum number of lens elements can be provided.