A zoom lens system includes a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group. Upon zooming, the first through fourth lens groups move toward the object so that the distance between the first and the second lens groups becomes longer, the distances among the second, the third and the fourth lens groups become shorter. The third lens group includes a positive 3-1st lens element, a positive 3-2nd lens element, and a negative 3-3rd lens element. The zoom lens system satisfies the following condition: -1.8<.O slashed.r.sub.3-3-1 /.O slashed.w<-1.1... (1) wherein .O slashed.r.sub.3-3-1 designates the surface power of the object-side surface of the negative 3-3rd lens element of the third lens group; and .O slashed.w designates the power of the entire lens system at the short focal length extremity.