Provided is a semiconductor device with a high breakdown voltage yield of a bipolar transistor and a high bandwidth and quantum efficiency of a light receiving element. An optical semiconductor device includes monolithically integrated transistor and light receiving element. The light receiving element includes a p-type semiconductor layer, an n-type epitaxial layer formed on the p-type semiconductor layer, and an n-type diffusion layer formed on the n-type epitaxial layer. An n-type impurity concentration of the n-type diffusion layer is 3×1018 cm−3 or less at a depth of 0.12 μm or more below a surface of the n-type diffusion layer, 1×1016 cm−3 or more at a depth of 0.4 μm or less below the surface, and 1×1016 cm−3 or less at a depth of 0.8 μm or more below the surface, and an interface between the p-type semiconductor layer and the n-type epitaxial layer is located at a depth of 0.9 μm to 1.5 μm below the surface.