PURPOSE: To avoid the occurrence of distortion in a receiver, by supplying the wave received at a receiving antenna to the receiver via an amplifier which has the gain to be controlled in accordance with the field intensity.

CONSTITUTION: The received wave given from a radio antenna which is used in common with a hot wire 2 of a rear window glass 1 is supplied to a high- frequency amplifier 21 and a controller 22. When the field intensity of the amplifier 21 exceeds 100dB, for example, a relay 23 is energized. Then the contact of the relay 23 is turned on, and a capacitor C1 is connected in parallel to the amplifier 21. As a result, the received wave is supplied to a receiving device 9 with no amplification. Thus no distribution occurs in the device 9.