A zoom lens system, in order from an object side to an image side, comprising a first lens unit of positive power, a second lens unit of negative power, a third lens unit of positive power, and a fourth lens unit of positive power, wherein the first lens unit is composed of three or fewer lens elements, wherein the second lens unit is composed of three lens elements, wherein in zooming, the first to the fourth lens units are moved individually along an optical axis such that air spaces should vary, so that variable magnification is achieved, and wherein the conditions are satisfied: 5.50≦fG1/fW≦7.92, ωW≧35 and fT/fW≧10 (fG1 is a composite focal length of the first lens unit, ωW is a half view angle at a wide-angle limit, and fT and fW are focal lengths of the entire system respectively at a telephoto limit and at a wide-angle limit), an imaging device and a camera are provided.