PURPOSE: To attain 2-way communication among plural automobiles by using 4 frequencies and to avoid confusion of a BGM or the like in an automobile with that of a communication opposite party by using one sound source in common.

CONSTITUTION: A voice signal collected by a microphone 8 of a slave set A is sent to a master set by using a weak radio wave whose frequency is F1Hz and synthesized by a voice synthesizer 6 from a connector 2 with an external sound source. Furthermore, a transmitter 7 sends the signal to an FM receiver in another automobile by using a weak radio wave whose frequency is F2 Hz. The sent voice is sounded in an automobile by using a speaker or the like in the automobile. Similarly the voice signal collected by a microphone 11 in a slave set B is synthesized with an external sound source at the master set and sent by using weak radio waves whose frequencies are F3, F4Hz and sounded in the automobile by using the speaker in the automobile. Furthermore, the two operations as above are simultaneously implemented.