A method for manufacturing a semiconductor optical device includes: forming a laminated semiconductor structure of GaN-based materials on a semiconductor wafer, the laminated semiconductor structure forming a laser diode of GaN-based materials, including an active layer having a quantum well structure; cleaving the semiconductor wafer including the laminated semiconductor structure to expose a cleaved end face of the laminated semiconductor structure; and forming an SiO2 film on the cleaved end face and performing a heat treatment to cause Ga vacancy diffusion in the active layer to disorder the quantum well structure of the active layer.