A zoom lens system includes, in the order from the object side to the image side thereof, a first lens group of positive refraction power, a second lens group of negative refraction power, a third lens group of positive refraction power, a fourth lens of positive refraction power, and an imaging plane. The first lens group and the third lens group are immovably mounted. The second lens group and the fourth lens group are movably mounted. The zoom lens system satisfies the formulae: 5