A zoom lens system according to the present invention, from an object side to an image side, comprises a first lens unit having positive optical power, a second lens unit having negative optical power, a third lens unit having positive optical power, and a fourth lens unit having positive optical power. In zooming, the first to the fourth lens units all move along the optical axis. The condition (8): 0.15 35; dG3 is an optical axial center thickness of the third lens unit; dG is a sum of the optical axial thicknesses of the first to the fourth lens units; ω is a half view angle at a wide-angle limit; fT is a focal length of the entire system at a telephoto limit; and fW is a focal length of the entire system at a wide-angle limit). As a result, the zoom lens system has a reduced size and still realizes a wide view angle at a wide-angle limit, as well as a remarkably high zooming ratio and high performance.