The semiconductor optical device is provided with an optical waveguide part and an optical amplification part respectively provided on the GaAs semiconductor substrate. The optical amplification part includes at least one semiconductor optical amplifier. The optical waveguide part includes optical elements including optical waveguides. The optical waveguides are optically connected to the semiconductor optical amplifier. The semiconductor optical amplifier is provided with an active layer including a Ga.sub.x In.sub.1-x N.sub.y As.sub.1-y semiconductor, a first conductive type clad layer and a second conductive type clad layer respectively with the active layer between them. The optical waveguides are respectively provided with a core semiconductor layer including at least either of a GaInNAs semiconductor or a GaAs semiconductor, a first clad semiconductor layer and a second clad semiconductor layer respectively with the core semiconductor layer between them.