To provide a substrate which gives high latitude for etching and which can be easily polished.

First, a single crystal silicon 1 is prepared and the surface is formed into concave by rough processing by a machine or the like (b). An amorphous silicon 2 is deposited by CVD method on the roughly processed surface of the single crystal silicon (c). After deposition of the amorphous silicon, the surface is polished by a ball feeding method or float polishing method, and then diffraction grating grooves are formed by Ar ion beam etching.