Providing a zoom lens system having excellent optical performance, an optical apparatus, and a method for zooming the zoom lens system. The zoom lens system includes, in order from an object, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having negative refractive power. Upon zooming from a wide-angle endstate W to a telephoto end state T, each distance between adjacent lens groups is varied. The second lens group G2 includes, in order from the object, a front group G2a having positive refractive power, and a rear group G2b. Focusing on the object is carried out by moving the front group G2a along an optical axis. Given conditional expressions are satisfied.