A diffraction optical element including an element body having a large number of grooves formed on its surface to form a grating, and a grating shape adjusting layer deposited on a grating surface of the element body by oblique incidence or material particles onto the grating surface, wherein the grating shape adjusting layer per se is made of a reflecting film material. By the grating shape adjusting layer, the contour shape of the resulting grating surface in a section perpendicular to a lengthwise direction of the grooves is made not similar to the sectional shape of the element body. A reflecting film may be formed on a grating shape adjusting layer made of another material. A reflecting film may be provided on the element body before a grating shape adjusting layer made of a transparent dielectric material is provided on the reflecting film.