PURPOSE: To simplify the constitution of the titled control system by performing a switch with no hit between a using circuit and a spare circuit to eliminate the omission of data and making use of a transmission circuit for bipolar signal.

CONSTITUTION: A switch circuit 1 of the transmission side and a switch circuit 2 of the reception side are connected to in-use circuits L1∼Ln and a spare circuit LA of a communication system via transmitters Ta and T1∼Tn and receivers Ra and R1∼Rn respectively. Then changeover switches Sa11∼Sa1n are provided to the circuit 1, and input terminals IN1∼INn of the transmission side are switched to the transmitter Ta connected to the circuit LA. While changeover switches Sa21∼Sa2n and S21∼2n are provided to the circuit 2 and switch the LA and L1∼Ln to connect them to output terminals OUT1∼OUTn of the reception side. These circuits 1 and 2 are controlled by circuit control circuits 3 and 4, and circuits 3 and 5 are connected via a control signal line 5. The switches Sa11∼Sa1n, Sa21∼Sa2n and S21∼Sn are controlled at prescribed time points by the switch commands given from circuits 3 and 5. Then the circuit LA is switched to circuits L1∼Ln with no hit.