The zoom lens system is used for forming an optical image of an object with a variable magnification, and, in order from the object side, comprises a first lens unit G1 of negative optical power, a second lens unit G2 of positive optical power and a third lens unit G3 of positive optical power. The lens units move respectively along the optical axis so that a magnification is changed with changing a distance between the respective lens units. The second lens unit G2 comprises at least three lens elements including an object side lens element of the second lens unit which is a positive lens element arranged on the most object side with the surface of strong curvature facing the object side, and an image side lens element of the second lens unit which is a positive lens element arranged on the most image side with the convex surface facing the object side.