PURPOSE: To perform audio communication via a branching filter, by connecting each of a plurality of stations N to different branching terminals of a branching filter with an analog carrier line and using a multi-frequency signal for a monitor signal.

CONSTITUTION: Each of three stations A, B, and C is connected to a different branching terminal of a branching device 4 with an analog carrier line 5 and audio communication is made between the two stations among the three stations via the device 4. The stations A, B and C respectively provide one private branch of exchange 2 and a carrier device 6. Each device 6 transmits non-communication signal in the frequency specific to the station itself at non-communication as a monitor signal to the line 5 and stops the transmission of the non-communication signal in the frequency specific to the station itself at call start and transmits the non-communication signal in the frequency specific to the connected opposing station after the stop. Thus, the connected opposing station is informed of being called that the transmission of non-communication signal in the frequency specific to the call start station is stopped and the transmission of the non-communication signal in the frequency particular to the own station is started after the stop.