PURPOSE: To obtain a transmission type blazed grating of high quality by coating a substrate with a negative type electron beam resist, irradiating the resulting resist film with electron beams, forming a relief grating which is ion-etched at a lower speed on the resist film, and carrying out ion etching using the grating as a shadow mask.

CONSTITUTION: A substrate 1 is coated with a negative type electron beam resist, the resulting resist film 2 is irradiated with electron beams, and the irradiated resist film 2 is coated with a photoresist which is ion-etched at a lower speed than the film 2. The resulting photoresist film 3 is formed into a relief type diffraction grating, and the film 2 is ion-etched with ion beams in an oblique direction to the substrate 1 using the diffraction grating as a shadow mask. The resist film 2 is made insoluble in solvent when irradiated with electron beams, and the film 2 is incompatible with the photoresist coating the film 2, so a satisfactory relief grating is obtd.