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.