An optical semiconductor device includes: an active region which includes an active layer which produces light when current is injected therein, a first diffraction grating layer having a first diffraction grating with a prescribed grating period, and a phase shift portion formed within the first diffraction grating layer, wherein the phase shift portion provides a phase shift not smaller than 1.5π but not larger than 1.83π; and a distributed reflection mirror region which is optically coupled to a first end of the active region as viewed along a direction of an optical axis, and which includes a second diffraction grating which reflects the light produced by the active region back into the active region.