A zoom lens system ZL mounted in an electronic still camera 1 etc is constructed to include, in order from an object side, a first lens group G1 having negative refractive power and a second lens group G2 having positive refractive power, in which the second lens group G2 has two cemented lenses. A distance between the first lens group G1 and the second lens group G2 changes when a lens position state changes from a wide-angle end state to a telephoto end state, thereby providing the zoom lens system exhibiting preferable optical performance, an optical apparatus including the zoom lens system and a magnification varying method using the zoom lens system.