An optical semiconductor device includes a wavelength-tunable semiconductor laser chip, a mount carrier, a first temperature sensor and a wire. The wavelength-tunable semiconductor laser chip has a first optical waveguide and a second optical waveguide. The second optical waveguide has a heater on a surface thereof and is optically coupled to the first optical waveguide. The mount carrier is for mounting the wavelength-tunable semiconductor laser chip, and has a first area arranged at a surface of the mount carrier of the first optical waveguide side when the wavelength-tunable semiconductor laser chip is mounted. The first temperature sensor is mounted on the first area. The wire couples between the heater and a second area arranged at a surface of the mount carrier of the second optical waveguide side when the wavelength-tunable semiconductor laser chip is mounted.