PURPOSE: To enhance the reliability of the control system, by securing the double link-type circuit connection among the main master station, the secondary master station and plural numbers of slave stations via the main and secondary circuits each and then carrying out the transfer of the data between the secondary master station and the slave station which detected the fault of the main circuit.

CONSTITUTION: The connection is secured between master station 11 and slave stations 121∼12N via main circuit 13, and at the same time each slave station is connected to secondary master station 14 via secondary circuit 15 to form the double link-type circuit. Slave station 12i (i=1∼n) possesses demodulator 21i to be connected to circuit 13, demodulator 22i to be connected to circuit 15, changeover connection switches 23i and 24i, and receiving circuit 25i each. In the normal state, the data is transmitted and received between the main master station and each slave station via circuit 13. However, in case circuit 13 has a fault between slave stations 122 and 123, slave station 123 and its subsequent stations cannot receive the signal from master station 11. In this connection, circuit 25i changes over switches 23i and 24i and then carries out the data transfer to and from master station 14 via circuit 15. The main master station stops the transmission after master station 14 confirmed the faulty area. At the same time, slave stations 121 and 122 also carry out the communication to station 14 through changeover to circuit 15.