A zoom lens system includes a first lens unit having positive optical power, a second lens unit having negative optical power, a third lens unit having positive optical power, and a fourth lens unit having positive optical power, the lens units being arranged in that order from an object side towards an image side. The second and fourth lens units are configured to move when the zoom lens system performs zooming. In this zoom lens system, the second lens unit consists of a negative lens element, a negative lens element, a negative lens element, and a positive lens element that are arranged in that order from the object side towards the image side. The focal lengths of the third and fourth lens units are set at appropriate values so that the zoom lens system can have a high zoom ratio while exhibiting high optical performance over the entire zoom range.