an optical semiconductor device with a semiconductor laser formed over a semiconductor substrate , and a modulator formed over the semiconductor substrate and continuously arranged with the semiconductor laser , wherein the semiconductor laser includes a first region having a diffraction grating with a phase shift , a second region arranged between the first region and the modulator , and in which the diffraction grating is not formed , and a common active layer formed over the first region and the second region , a first electrode injecting a current into the common active layer .