PURPOSE: To use effectively communication channels of a loop transmission line, by using a single channel to realize broadcasting communication.

CONSTITUTION: In the normal communication, receiving data DR is received by a receiving buffer 51 and is led into a terminal 2. Transmission data DS is transmitted onto the transmission line after stored in a transmission buffer 52 from the terminal 2. Meanwhile, in the broadcasting communication, a broadcasting communication control signal S becomes high to take receiving data DR into the receiving buffer 51, and a gate 41 is opened to transmit data as transmission data DS onto the transmission line. At this time, a gate 43 is closed, and therefore, information from terminals is not transmitted. Thus, data transmitted from a broadcasting communication request terminal is transferred on the loop transmission line without disappearing in each exchange, and thus, the broadcasting communication on a signal channel is possible.