A zoom lens system with a focusing system other than a front group focusing system is provided. The lens system comprises a first lens group including at least two subgroups with a variable air space therebetween to be varied during zooming, and a focusing lens group located at the image side of the first lens group, the lateral magnification .beta..sub.F with respect to the focusing lens group being continuously variable during zooming with a condition,.vertline..beta..sub.F .vertline.>1always fulfilled so that .vertline..beta..sub.F .vertline. is increased in accordance with the increase of the focal length of the whole lens system. The focusing lens group may be divided into two or three sub-groups with a variable air space therebetween to be varied during zooming. Further, an additional lens group may be located at the image side of the focusing lens group.