The present invention provides a semiconductor optical device 1 that includes a photodiode capable of monitoring front light of the light-emitting device. The optical device 1 includes a photodiode 3, which is formed on a substrate 7 and has an absorption layer 9 and a semiconductor layer 11, and an optical waveguide 5 also formed on the substrate 7. The optical waveguide 5 includes, in addition to the active layer 13, two semiconductor layers 15 and 17. A portion of the light from the active layer 13 enters the photodiode 3 reflected by the end surface 5a of the waveguide 5 and the rest portion of the light passes through the end surface 5a and outputs therefrom. In the present invention, the photodiode 3 and the waveguide 5 are integrally formed on the substrate 7.