An optical semiconductor device includes an optical semiconductor element, a metal pattern and at least one thermal conductive material. The optical semiconductor element has a first optical waveguide region and a second optical waveguide region. The second optical waveguide region is optically coupled to the first optical waveguide region and has a heater for changing a refractive index of the second optical waveguide region. The metal pattern is provided on an area to be thermally coupled to a temperature control device. The thermal conductive material couples the metal pattern with an upper face of the first optical waveguide region of the optical semiconductor element. The thermal conductive material is electrically separated from the first optical waveguide region.