A transmission type optical element comprising a first hologram having a first surface and a second surfaces and a second hologram having a third surface and a fourth surface. The first surface of the first hologram and the third surface of the second hologram are oppositely arranged. A first light beam having a first wavelength is made to enter the first hologram from the side of the second surface. It is transmitted through the first hologram and exits the first hologram from the side of the first surface. The light beam then enters the second hologram from the side of the third surface. It is diffracted by the second hologram and exits the second hologram from the side of the third surface. The light beam then enters the first hologram from the side of the first surface and is diffracted by the first hologram.