A method of manufacturing an optical semiconductor device includes: forming a mesa structure having an n-type cladding layer, an active layer and a p-type cladding layer in this order on a substrate; forming a p-type semiconductor layer on a side face of the mesa structure and a plane area located at both sides of the mesa structure, the p-type semiconductor layer having a thickness of 5 nm to 45 nm on the plane area; and forming a current blocking semiconductor layer on the p-type semiconductor layer so as to bury the mesa structure, a product of the thickness of the p-type semiconductor layer and a concentration of p-type impurity of the p-type semiconductor layer on the plane area being 2.5×1019 nm/cm3 or less.