A zoom lens system includes a first lens unit having a positive optical power, a second lens unit having a negative optical power which moves during zooming, a third lens unit having a positive optical power which does not move for zooming, and a fourth lens unit having a positive optical power which moves during zooming. The first lens unit, the second lens unit, the third lens unit, and the fourth lens unit are arranged in order from the object side to the image side. A back focus of the zoom lens system and a focal length of the third lens unit are adequately set.