PURPOSE: To obtain an optical semiconductor device which is capable of oscillating laser rays by a low threshold current and independent of polarization in photodetective characteristics, wherein the optical semiconductor device is available to a laser transceiver type optical communication system.

CONSTITUTION: A semiconductor laser part 7a equipped with an active layer 2a of tensile strain quantum well structure and capable of oscillating in a TM mode and a TE mode photodetective part 7b equipped with a photodetective part 2b of compressive strain quantum well structure are arranged in series in a direction in which light resonates and formed into a monolithic structure as kept in a state that they are electrically isolated from each other. By this setup, the active layer 28 of the semiconductor laser part 7a and the photodetective part 2b of the TE mode photodetective part 7b are so arranged as to match to each other, light of TM mode is absorbed in the semiconductor laser part 7a, and light of TE mode is absorbed in the TF, mode photodetective part 7b, whereby an optical semiconductor device of this constitution is set free from polarization dependence.