The presence or absence and the intensity of refractive index distribution are easily controlled with high reproducibility without depending on the fabricating process accuracy. InGaAs well layers (14a) and (14b), which have a narrow bandgap and a high refractive index, are enclosed by a lower barrier layer (13), an intermediate barrier layer (15), an upper barrier layer (16) and a buried layer (18) of GaAsN-based materials of a wide bandgap. Then, by adjusting the nitrogen crystal mixture ratio of the GaAsN-based materials that constitute the barrier layers (13), (15) and (16) and the buried layer (18), the presence or absence and the intensity of the refractive index distribution are controlled. Thus, the refractive index distribution is easily controlled with high reproducibility without considering the configuration of a diffraction grating (17), a refractive index balance with respect to the buried layer (18) and so on, i.e., without depending on the fabricating process accuracy.