A zoom lens system comprises from the object side to the image side: a first lens unit of a positive refractive power; a second lens unit of a negative refractive power; and a third lens unit of a positive refractive power; wherein one of the first and third lens units is shiftable along the optical axis of the lens system in both of the zooming operation and the focusing operation, and the second lens unit is also shiftable along the optical axis of the lens system in the zooming operation.
