Si atoms obtained by thermal decomposition of SiH4 are adsorbed in advance on one surface of a semiconductor substrate and side surfaces of a semiconductor mesa part. Thereby, prior to the growth of a buried layer, a diffusion protection layer composed of Si-doped InP with high impurity concentration is formed. As a result, when the buried layer is grown, Zn diffusing from an upper cladding layer is trapped by the diffusion protection layer, and interdiffusion between Zn and Fe is inhibited. Since the diffusion protection layer is formed uniformly at a small thickness of several monolayers, the diffusion protection layer is also inhibited from becoming a current leakage path. Consequently, the reliability of the semiconductor optical device can be improved.