An object is to provide an ultra-compact zoom lens system suitable for a video camera and a electronic still camera using a solid-state imaging device and the like with securing high optical performance. The zoom lens system includes, in order from an object along the optical axis, a first lens group having negative refractive power, a second lens group having positive refractive power, a third lens group having positive refractive power, and at least one lens group having positive refractive power. The first lens group is fixed upon zooming from a wide-angle end state to an telephoto end state and focusing. A plurality of lens groups except the first lens group are moved upon zooming from the wide-angle end state to the telephoto end state, respectively. A plurality of lens groups except the first lens group G1 are moved upon focusing, respectively. Given conditional expressions are satisfied.