An optical semiconductor device is provided as one achieving reduction of power in phase control. The optical semiconductor device has: a first optical waveguide having a plurality of segments each of which has a diffraction grating region with a diffraction grating and a space portion coupled to the diffraction grating region, having two ends interposed between the diffraction grating regions, and having a constant optical length, wherein at least one of the segments is provided with a phase shift structure; a first phase control device for adjusting a phase of light in each segment with the phase shift structure; and a second phase control device for adjusting a phase of light in each segment without the phase shift structure.