A semiconductor optical device includes a GaAs substrate of a first conductivity type; a III-V compound semiconductor layer provided on the GaAs substrate; an active layer provided on the III-V compound semiconductor layer; and a cladding layer of a second conductivity type provided on the active layer, wherein the band gap energy of the III-V compound semiconductor layer is larger than the band gap energy of the GaAs substrate, wherein the band gap energy of the active layer is smaller than the band gap energy of the GaAs substrate, and wherein the thickness of the III-V compound semiconductor layer is not more than 0.2 μm.