Abstract: A heat sink element in the form of a coil with a high thermal conductivity is formed. One end of the heat sink element is directly connected to a circuit element connection conductor under a high potential in a high-frequency circuit mounted on an electronic circuit substrate, and the other end thereof is directly secured to a metal case having the electronic circuit substrate contained therein. As a result, the heat sink element serves as a heat channel transferring the heat generated due to the temperature increase in the circuit element connection conductor to the metal case by means of thermal conductivity.