Abstract: In a quarter-wavelength coupled line type directional coupler including a first dielectric layer having first and second surfaces parallel to each other, a ground plane conductor is formed on the first surface of the first dielectric layer, and two coupled microstrip conductors each having a quarter wavelength are formed on the second surface of the first dielectric layer, arranging close to each other so as to be electromagnetically coupled with each other. Further, a second dielectric layer is formed on the second surface of the first dielectric layer, on which the coupled microstrip conductors are formed, and a floating potential conductor is formed on the second dielectric layer, arranging close to the microstrip conductors so as to be electromagnetically coupled with the coupled microstrip conductors. Then a cut portion is formed in the ground plane conductor so that the ground plane conductor is separated apart from the coupled microstrip conductors by a predetermined distance.