PURPOSE: To reduce the status detecting time of other stations belonging to one system by detecting the status of all stations in a minimum cyclic number of times.

CONSTITUTION: In transmitting data of the same capacity from one station S1 to other stations S2∼S4, the station S1 transmits a frame signal to a transmitter L. The stations S2∼S4 write the status of the own station to a status signal area STATUS assigned to the own station succeeding to a frame identification bit F of the frame signal and transmit it to the next stations S3, S4 and S5 with relay. Thus, when this frame signal returns to the original station S1, the status of all the stations S2∼S4 is detected.