PURPOSE: To perform efficiently communication between arbitrary device out of all transmission device connected into a loop, by detecting a transmission station indication signal, a receiving station designation signal, and existence of this designation signal.

CONSTITUTION: The receiving signal from terminal 110 is written in memory 22, and device discrimination signal receiving circuit 25 decides whether receiving station designation signal R out of the receiving signal supplied from memory 22 is a signal designating its own device, a signal designating another device, or an idle device designation signal, and circuit 25 decides the transmission device from which trasnsmission station indication signal S is transmitted. when the local signal is output, information signal receiving circuit 28 outputs information signal D to terminal 180. When the transmission request signal is given to terminal 190 and the local or idle signal is given from terminal 252, receiving device designation signal transmission circuit 27, transmission device indication signal transmission circuit 26, and information signal transmission circuit 29 transmit receiving station designation signal R, display signal S, and information signal D to connecting circuit 30 respectively.