A deflection optical element, which diffracts incident light, includes a substrate having translucency, and a holographic material layer disposed so as to overlap the substrate, the holographic material layer being formed with a diffraction grating composed of interference fringes, wherein the holographic material layer is formed with an alignment mark where the interference fringes are discontinuous, and the alignment mark is located in an optically effective area where the holographic material layer diffracts the incident light.