The invention provides a zoom lens system which achieves a wide angle of field, has a simplified construction, and is suitable for use on video cameras. The zoom lens system comprises, in order from an object side thereof, a first lens group G1 having positive refracting power, a second lens group G2 having negative refracting power, a third lens group G3 having positive refracting power, and a fourth lens group G4 having positive refracting power. The second lens group G2 moves toward an image side thereof while the third G3 and fourth G4 lens groups move constantly toward the object side for zooming from a wide-angle end thereof to a telephoto end thereof. The first lens group G1 consists only of a positive single lens.