The invention provides a zoom lens system that is reduced in terms of both size and cost. The zoom lens system comprises a first positive lens group G1, a second negative lens group G2, a stop S, a third positive lens group G3 and a fourth positive lens group G4. During zooming from a wide-angle end to a telephoto end of the system, while the first lens group G1 remains fixed, the second lens group G2 moves from an object side to an image plane side of the system, the third lens group G3 moves from the image plane side to the object side, and the fourth lens group G4 moves to keep an image plane at a constant position. The first lens group G1 is made up of a negative meniscus lens convex on an object side and a double-convex lens, the second lens group G2 is made up of a double-concave lens and a positive meniscus lens convex on an object side thereof, with an aspherical surface being used for a surface of the positive meniscus lens that is located on an image side thereof, the third lens group G3 is made up of a double-convex lens and a negative meniscus lens convex on an object side thereof, with an aspherical surface being used for a surface of the double-convex lens that is located on an object side thereof, and the fourth lens group G4 is made up of one double-convex lens.