PURPOSE: To obtain a blazed grating of a transmission type having high quality by forming a grating having roughly a rectangular section on the 2nd org. high polymer film and ion-etching the 1st org. high polymer film from a diagonal direction with said rectangular grating as a shadow mask.

CONSTITUTION: A photoresist film 6 is etched with an argon ion beam with a relief grating formed thereon as a mask by which a grating having roughly a rectangular section is formed on a gold layer 5 having a higher etching rate than the film 6. The 2nd org. higher polymer layer 4 having a higher etching rate than the gold layer is etched with an oxygen ion beam with the grating of the layer 5 as a mask to form a grating having a roughly rectangular section. The remaining layer 6 and layer 5 are removed and thereafter with the rectangular grating as a shadow mask the film is ion-etched with argon ion or oxygen ion from the direction shown by an arrow 7, by which the blazed grating of a transmission type having high quality is obtd.