A semiconductor device including a semiconductor optical amplifier in which a photodetector is integrated without causing power loss at a low cost. The semiconductor device includes a semiconductor substrate, a semiconductor optical waveguide, at least partly capable of functioning as a semiconductor optical amplifier, to guide signal light, and a photodetector provided in a region other than a region where the semiconductor optical waveguide is provided, wherein the semiconductor optical waveguide and the photodetector are integrated on the semiconductor substrate.