An optical semiconductor device comprises a distributed Bragg reflector layer of a first conductivity type, an optical absorption layer, and a semiconductor layer of a second conductivity type, sequentially formed on a semiconductor substrate; wherein said Bragg reflection layer of the first conductivity type has first semiconductor layers having a band gap wavelength larger than the wavelength of incident light, and second semiconductor layers having a band gap wavelength smaller than the wavelength of incident light; and an optical layer thickness of each of said first semiconductor layers is larger than the optical layer thickness of each of said second semiconductor layers.