PURPOSE: To obtain a blazed grating distributed periodically in a one-dimensional direction by curving a substrate in the periodic direction of the grating of a grating mask pattern formed on the surface of the substrate and by subjecting the whole surface of the substrate to ion milling with parallel ion beams emitted from a position above the mask in one direction.

CONSTITUTION: A substrate 23 is curved in the periodic direction of the grating 25W29 of a grating mask pattern formed on the surface of the substrate 23, and the whole surface of the substrate 23 is subjected to ion milling with parallel ion beams 24 emitted from a position above the mask in one direction to pattern the surface of the substrate 23 to be worked. By the curving before the ion milling, the angle of incidence of ion beams on the substrate 23 is changed every grating period, so the optimum angle of blazing to the periodic distribution of the grating can be attained by properly selecting the curvature of the surface 22 of a substrate cooling plate 21 contacting with the substrate 23 and the angle of incidence of ion beams.