A zoom optical system provided, in order from a reduction side toward a magnification side, with: a first lens unit having positive optical power; a second lens unit having positive optical power; a third lens unit having positive optical power; and a fourth lens unit having negative optical power; wherein the interval between adjacent ones of the first, second, third and fourth lens units is varied during zooming, and in an entire zooming range, a magnification side conjugate position relative to a reduction side conjugate position, and the position of the pupil of the zoom optical system relative to the reduction side conjugate position are substantially immovable.