An object is to provide a high-resolution zoom lens system which has fewer lens elements and therefore, can be retracted into a compact form when not in use; and the zoom lens system, in order from the object side, comprises a first lens unit G1 having negative power, a second lens unit G2 having positive power and a third lens unit G3 having positive power, wherein magnification is varied with change in intervals between each lens unit by independently moving each lens unit along an optical axis, in which the first lens unit G1 comprises one negative lens element L1 and one positive lens element L2, the second lens unit G2 comprises only a cemented lens element having set of three lens elements L3, L4 and L5 which are cemented with each other, and the third lens unit G3 comprises one positive lens element L6.