To provide a semiconductor optical device which can restrain laser characteristics from being deteriorated by excitation in a substrate mode and can reduce the number of manufacturing steps. A semiconductor optical device comprises a first DBR layer, provided on a semiconductor substrate, having first and second semiconductor layers stacked alternately, a first cladding layer, an active layer, and a second cladding layer. The semiconductor substrate has a bandgap higher than that of the active layer. The first DBR layer is transparent to light having an emission wavelength, while the first and second semiconductor layers have respective refractive indices different from each other. Since the first DBR layer is thus provided between the semiconductor substrate and first cladding layer, the guided light reaching the lower end of the first cladding layer, if any, is reflected by the first DBR layer, whereby light can be restrained from leaking to the semiconductor substrate. This can avoid the substrate-mode excitation, thereby suppressing its resulting laser characteristic deteriorations such as destabilization of oscillation wavelengths.