PURPOSE: To comfortably hear desired transmitting sound by providing a beat sound judging part to detect beat sound due to higher harmonic wave component of an oscillating circuit of sound output of a receiver installed on a vehicle, inputting this output to a frequency converting part, and changing the oscillating frequency of a transmitting circuit.

CONSTITUTION: When beat sound is generated in output of a receiver 8 installed on a vehicle, the beat sound is detected with a microphone 13 and judged with a beat sound judging part 12. The output of the judging part 12 is output to a frequency converting part 11, the transmitting frequency of a transmitting circuit 2 is changed by the output of the converting part 11, and the receiving frequency of the receiver is reset to the frequency not to generate beat. As a result of this, beat sound is not generated from a speaker 10, but received sound is only heard. In this way, at detecting beat sound in the receiver 8, the transmitting frequency of the transmitting circuit 2 is changed to perfectly eliminate the beat sound.