A zoom lens optical system includes a first optical group with a first lens having negative refraction, a second lens having positive refraction, and a light path changing unit to change the path of an incident light passing through the first lens to have light incident on the second lens. A second optical group is provided for varying magnification. The second optical group has a third lens having negative refraction, a fourth lens, and a fifth lens, wherein the fourth and fifth lenses are integrally connected and have negative refraction. A third optical group includes an iris diaphragm, a sixth lens having positive refraction, and a seventh lens having negative refraction. A fourth optical group is provided for focusing. The fourth optical group has an eighth lens and a ninth lens being integrally connected and both having positive refraction.