The present invention is directed to improve prior art zoom lens systems that have groups of lens pieces moved within a lens barrel for the zooming and the focusing and to provide an improved zoom lens system that is simplified and robust in configuration so as to withstand malfunctions well and is capable of effectively and assuredly eliminating stray light. The zoom lens system comprises a first cam barrel rotating about the optical axis and a second cam barrel located inside the first cam barrel in cam-linkage with the same, and the zoom lens system is characterized in that the second cam barrel has openings to let light beams pass through, and that a light-shielding plate like an iris diaphragm is supported by the second cam barrel in position closer to the photographed subject in the second cam barrel so as to shield the light beams that come in from the openings in the second cam barrel after going out of the openings in the second cam barrel and then being reflected from an inner surface of the first cam barrel.