A semiconductor optical device includes at least a lower cladding layer formed on a semiconductor substrate, a core layer formed on the lower cladding layer, and an upper cladding layer formed on the core layer. The core layer includes a first core layer of a material susceptible to oxidation and a second core layer of a material unsusceptible to oxidation, the first core layer and the second core layer being connected in sequence in an optical propagation direction. The second core layer is formed at a facet where a light is input or output.