A method of manufacturing directional coupler includes forming a first waveguide over a substrate, forming a separate section on respective ends of the first waveguide, and forming a second waveguide stacked over the first waveguide, wherein the first waveguide and the second waveguide are patterned by ink-jet process. The first waveguide and the second waveguide may be formed so as to overlap each other. The method may further include forming a projecting portion on the substrate, wherein the first waveguide being formed on the projection portion.