PURPOSE: To obtain a diffraction grating having a protective film, light in weight and causing no defect, by uniformly coating the surface of the diffractive grating obtd. by forming a wavy relief on the surface of a light-transmitting resin, with a transparent material having a refractive index similar to said resin in such a thickness to keep that relief form.

CONSTITUTION: A relief diffraction grating 2 made of light-transmitting resin having uniform refractive index is formed on a transparent flat glass plate base 1 or the like. The diffraction grating is obtained efficiently by forming interference fringes on a photoresist especially by the holographic technique. A protective film 14 made of transparent material having refractive index near that of the used resin is formed on the surface of the diffractive grating 2 in a uniform thickness so as to keep this relief form. For example, when the photoresist having 1.6W1.7 refractive index is used, polymono-chloroparaxylene having 1.64 refractive index is vapor deposited. SiO2 may be vapor deposited in accordance with refractive index of the grating 2. A light and uniform protective film 14 is thus formed, and trouble of crushing the relief face or causing stress strain is prevented.