A method of manufacturing an optical semiconductor device including: forming a mesa structure including a first conductivity type cladding layer, an active layer and a second conductivity type cladding layer in this order on a first conductivity type semiconductor substrate, an upper most surface of the mesa structure being constituted of an upper face of the second conductivity type cladding layer; growing a first burying layer burying both sides of the mesa structure at higher position than the active layer; forming an depressed face by etching both edges of the upper face of the second conductivity type cladding layer; and growing a second burying layer of the first conductivity type on the depressed face of the second conductivity type cladding layer and the first burying layer.