Method of manufacturing an optical device with a groove accurately formed

A first layer (2) and a second layer (3) are formed on a substrate (1). The first layer is made of a resist against a groove-sculpturing etchant used in etching the substrate while the second layer is made of an anti-corrosive material against dry etching. The second layer is at first patterned into a patterned second layer (3a) in the form of a groove-sculpturing mask pattern (8). With the patterned second layer used as a mask, the first layer is etched and patterned into a patterned first layer (2a) in the form of the above-mentioned mask pattern. With the patterned first layer used as a mask, the substrate is etched to form a groove (9).