A method of forming a linear grating is disclosed. When forming a first resist pattern covering certain surface regions of a substrate, the mask pattern position is shifted and the first resist pattern is formed such that the trench in the target region is completely filled with the first resist pattern even when an error in positioning occurs. The surface of the first resist pattern is etched, and a lower resist pattern is left to the same level as the uppermost step of the silicon substrate. On top of this, an upper resist pattern having the same pattern as the first resist pattern is formed. At this time, the mask pattern position is shifted and the exposure dose is adjusted such that one edge of the upper resist pattern is positioned on the lower resist pattern, and the other edge is positioned in a prescribed region border portion. The lower resist pattern and upper resist pattern are used as a mask to etch the silicon substrate.