PURPOSE: To enhance diffraction efficiency by forming a relief layer of no refractive index and a coat layer of (n) refractive index flat on the surface with respect the wavelength of light incident on the diffraction grating, and specifying the refractive indices no and (n).

CONSTITUTION: The relief layer 2 made of transparant resin changed in thickness like a sine wave is formed on a transparent flat glass 1, and a coat layer 3 made of transparent resin of (n) refractive index is formed on the layer 2, with the surface 4 of the layer 3 flattened. The diffractive grating thus obtained is raised in diffraction efficiency, with its depth kept constant, by setting the difference of refractive index between the layers 2, 3. The diffraction efficiency can be raised by forming the layer 2 of no refractive index and the layer 3 of (n) so as to satisfy n>2no-1.