PURPOSE: To realize priority control without redundancy with time by allowing each communication station to transmit a data in a specific frequency among frequencies f1∼fn different in priority order and allowing a communication station having the highest priority to continue communication when collision occurs.

CONSTITUTION: The priority of the communication stations T1∼Tn is discriminated depending on assigned carrier frequencies different from each other. When a signal is outputted on a carrier in a frequency fi from the communication station Ti, the signal is received/demodulated by a head end device HE, and a demodulated data RDi and a carrier detecting signal CDi are inputted to a priority selecting circuit 9. The data RDi appears at an output SD of the circuit 9. The signal is outputted in frequency f0 by a modulation circuit 10 and a transmission circuit 11. This signal is received by all the communication stations. The communication station Ti compares the transmitted data SDi and the said received data RD to recognize the presence of collision. When the signal is transmitted at the same time from plural communication stations, a pair of the data RD of each communication station and the signal CD is inputted respectively to the priority selecting circuit 9, and the circuit 9 selects and outputs the receiving data in the carrier frequency having the high priority.