An optical semiconductor device having a low threshold current and easiness of a single transverse mode oscillation is provided. The optical semiconductor device has a low device parasitic capacitance that allows a direct modulation at high speed. The optical semiconductor device comprises a first conduction type substrate, a stripe shaped active layer formed on the first conduction type substrate, a mesa shaped burying layer formed around the active layer and having a larger band gap than that of the active layer, and a groove that electrically isolates the burying layer, wherein the section of the burying layer is in an inverse trapezoid shape of which the upper base side is longer than the lower base side.