PURPOSE: To form different diffraction gratings with the same phase relation on one substrate by a method wherein, after a thin film of compound containing InP is formed in a region of an InP substrate where the first diffraction grating is to be formed, a resist film is formed over the whole surface.

CONSTITUTION: After an InGaAsP layer 22 is formed on the surface of an InP substrate 21, the InGaAsP layer 22 is removed by etching except the part on a region where the first diffraction grating is to be formed. Then a resist film 23 is formed over the whole surface and the pattern of the diffraction grating is formed by a double-beam interference method. If etching is carried out with sodium hydroxide after exposure, only the InGaAsP layer is etched. Then, if etching is carried out with a solution of hydrochloric acid, where the InGaAsP layer remains, this layer serves as a mask and prevents the substrate from being etched and, on the other hand, where the InGaAsP layer is not left, only the resist serves as a mask and the substrate is etched. As a result, two different etched shapes are formed on the substrate so that the first diffraction grating 24 and the second diffraction grating 25 can be obtained. Then, the resist film and the InGaAsP layer are removed.