A zoom optical system includes in order from a reduction side to a magnification side: a first lens unit having a positive optical power; a second lens unit having a negative optical power; a third lens unit having a positive optical power; and a fourth lens unit having a positive optical power, wherein respective intervals between the above described first, second, third and fourth lens units vary during zooming; a magnification-side conjugate position with respect to a reduction-side conjugate position and a position of a pupil of the above described zoom optical system with respect to a reduction-side conjugate position are substantially immobile respectively across the entire zooming range, and the above described fourth lens unit moves toward the magnification side and an interval between the above described first lens unit and the above described fourth lens unit increases during zooming from a wide-angle end to a telephoto end.