PURPOSE: To form an adequate diffraction grating by a method wherein all the surface of a wafer is set uniform in light exposure time.

CONSTITUTION: Laser rays of prescribed wavelengths emitted from a single laser beam source are split. Split laser rays L1 and L2 are made to obliquely irradiate a semiconductor wafer, on which a resist film 9 is deposited, from the opposite directions, whereby interference fringes 11 are formed on the resist film 9 through laser interference and exposed, and the exposed resist, film 9 is enveloped. At this point, before the resist film 9 is developed, the interference fringes 11 are partially formed on the resist film 9, and a part of the resist film 9 other than the interference fringes 11 is fully exposed to laser rays of a single source. The resist film 9 is gradually lessened in fully exposed region in a peripheral part 10b.