PURPOSE: To miniaturize a device and ensure a fabrication method of a diffraction grating unlikely to be affected by mechanical vibration.

CONSTITUTION: A groove (10) of a desired grating pattern is formed on the lower surface of a glass substrate (8), and the surface of the glass substrate (8) where the groove is formed is disposed to make contact with a resist (2) application surface of a wafer (1) in a confronting relation. The glass substrate (8) is irradiated with an exposure light ray (9) from the upper surface thereof, whereby the exposure light ray (9) takes light intensity distribution on the bottom of the glass substrate (8) corresponding to the presence of the groove (10). With the light intensity distribution, a grating pattern part is formed on the resist (2).