PURPOSE: To simplify the constitution of an interphone device, by using local oscillators of the same oscillating frequency at the demodulating part of the center and terminal sides respectively and at the same time using the same local oscillator for modulation and demodulation at the center side.

CONSTITUTION: For transmission of an audio signal to the center side from a terminal side, a carrier wave is transmitted by FM modulation 72 with an audio signal containing an amplified carrier wave. At the center side an intermediate frequency signal is obtained by mixing a received high-frequency signal with the output of a local oscillator 94 by a mixing circuit 100. While the audio signal is transmitted to the terminal side form the center side in the form of a high-frequency signal obtained by mixing a signal obtained by giving double multiplication 95 to the output signal of the oscillator 94 and an FM carrier wave obtained by an audio signal through a mixing circuit 93. At the terminal side an intermediate frequency signal is obtained by mixing the received high frequency signal with a signal obtained by giving double multiplication 79 to the output of a local oscillator 78 through a mixing circuit 77. The oscillators 78 and 94 use the same oscillating frequency. As a result, the circuit constitution is simplified with a reduction of the number of crystal oscillators for an interphone device.