To enable simultaneous speech between arbitrary plural transmitters by making a master station output only a sound signal of its own station to a transmission line, making a slave station add the sound signal of its own station to the sound signal from the transmission line and output them.

A transmission line for transmitting a sound signal in the inverse direction to each other is provided, a master station outputs only the sound signal of its own station to a transmission line, and a slave station receives a signal that does not include the sound signal of the station in use and adds to this the sound signal of its own station. With regard to a sound signal running a 0 system transmission line 10, it is a signal belonging to an A station that is sent to a B station, it is a signal adding the signal belonging to the A station and the one belonging to the B station that is send to a C station and it is a signal adding the signals of the A station, the B station and the C station that is sent to a D station. Thus, each station only has to straightforwardly add to each received signal the sound signal of its own station. The A station straightforwardly fetches a received signal and at the same time when it sends the sound signal of its own station to the 0 system transmission line, other sound signals are not included. The similar processing is executed to a 1 system transmission line 11.