Provided is a zoom lens system suitably used for, for example, an image taking optical system of a camera including a solid-state image pickup element. The zoom lens system includes a first lens unit having negative optical power, a second lens unit having positive optical power, and a third lens unit having positive optical power. During zooming, an interval between the first lens unit and the second lens unit and an interval between the second lens unit and the third lens unit are changed. The first lens unit consists of a first lens element having negative optical power and a second lens element having positive optical power. The first lens element has an aspherical surface. The first lens element is made of a material having a high refractive index.