PURPOSE: To ensure the fault detection of the down-circuit as well as the presumption of the faulty area, by transmitting the signals from several slave stations to the master station via the up-circuit and then connecting the slave station alternately to the main down-circuit and the secondary down-circuit for reception of the signals from the master station.

CONSTITUTION: The odd-numbered slave stations 121, 123...12n-2, 12n are connected to main down-circuit 16; while even-numered slave stations 122, 124...12n-3, 12n-1 are connected to secondary down-circuit 18 respectively. And these slave stations receive the signals from the master station each. And if the disconnection of circuit 18 occurs between slave stations 12n-5 and 12n-3, master station 11 can knows the fault of circuit 18 from the fact that no answer is given from slave stations 12n-3 and 12n-1. At the same time, the signals of the circuit disconnection are delivered from slave stations 12n-3 and 12n-1 based on the fact that the signal of a prescribed level does not exist in receiving circuit 121. And logic part 126 delivers the switching command of switch 123 to connect slave stations 12n-3 and 12n-1 to circuit 16. Thus the communication is started again between the master station and the slave stations. Slave stations 12n-3 and 12n-1 send the information of the disconnection of circuit 18 to have the confirmation at the master station.