PURPOSE: To ensure the satisfactory function of a directional coupler with a small and rigid structure, by forming a coil of the optional number of turns with an insulated lead wire centering on a conductor belt connecting the output stage of a transmitter and a load terminal and a substrate part which supports said conductor belt.

CONSTITUTION: A conductor belt 13 and other desired patterns are formed on a copper plate 11A of a two-sided substrate 10 by an etching process. Then holes 14a∼14n are drilled through the substrate 10 at both sides of the belt 13, and a conductor 12 is wound like a solenoid through these holes and centering on the belt 13. An end of the conductor 12 is connected to a detecting rectifier 15, and the other end is grounded via a balance controller 16. It is desirable to connect strictly between the conductor 12 and the belt 13. In this connection, a coated electric wire is used with a direct contact with the coil. Otherwise, the conductor 12 is wound with a thin insulator held between the surface of the substrate 10 and the conductor 12.