PURPOSE: To provide a method of forming a diffraction grating for obtaining a multiwavelength laser array consisting of lasers, which respectively have different luminous wavelength.

CONSTITUTION: A process, wherein an optical mask 14 having a plurality of parts with a film thickness to change gradually with different angles is applied to an optical system of a two-luminous flux interference aligner and a substrate- shaped resist is exposed with two-luminous flux interference light using the aligner, a process, wherein the resist subsequent to the exposure is developed to form a resist pattern, and a process, wherein a substrate is subjected to etching treatment using the resist pattern as a mask, are provided and a diffraction grating having different periodic parts is formed on the same substrate. A multiwavelength laser array can be simply formed.