PURPOSE: To decrease the hit frequency by detecting not only a circuit fault but a fault of the input signal and deciding the circuit fault from the results of both detections.

CONSTITUTION: Only detection signals for circuit faults given from detection circuits 12, 14, 22, 24 and 26 are supplied from a MODEM (modulator/demodulator) A during operation of the MODEM A. In such a case, a fault of the MODEM A itself is decided. Then a switch control signal Q is outputted to changeover switches 30 and 32 from a switch deciding circuit 108. Thus the MODEM A is switched to a MODEM B. In case a switch stop signal is supplied or a circuit fault is detected at the side of the MODEM B, the circuit 108 out no signal Q. Thus the MODEM A is not switched to the MODEM B.