A zoom optical system includes, in order from the reduction conjugate side to the magnification conjugate 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 negative optical power, wherein in an entire zoom range, a magnification-side conjugate position relative to a reduction-side conjugate position, and the position of a pupil of the zoom optical system relative to the reduction-side conjugate position are each substantially fixed, and the interval between the first lens unit and the second lens unit at a telephoto end is larger than the interval between the first lens unit and the second tens unit at a wide-angle end.