A method of manufacturing a broadside-coupled directional coupler including a main line and a sub line which are broadside-coupled to each other is provided. The method includes at least two adjustment processes. A line width of a coupled line part of the sub line on a first surface of a dielectric substrate is adjusted wherein the dielectric substrate having a second surface being opposite to the first surface, and the second surface has a ground plane. A position of a boundary portion where the ground plane and the coupled line are in contact with each other is adjusted.