A zoom lens system comprising an afocal front lens assembly and an image forming rear lens assembly is provided with stabilized optical means arranged in the optical path between said front and rear lens assemblies. When zooming, the stabilized optical means is controlled in accordance with the variation of angular magnification of the front lens assembly and with the deviation of the housing incorporating the zoom lens system from an original line of sight. The image is thus stabilized at the focal plane thereof against such deviation by varying the deflecting angle of the exiting light axis from the stabilized optical means with respect to the entering light axis. In one embodiment of the present invention, the rear lens assembly includes one or more members movable for zooming in such a manner that the zooming operation of the front lens assembly is consecutively followed by the zooming operation of the rear lens assembly, or vice versa. The stabilized optical means is preferably constructed in the form of a variable optical wedge, or a combination of a swingable mirror and a fixed mirror.