PURPOSE: To detect the delay of the transmission device or the like of the remote side and then to inform it to the operator through generation of alarm by breaking the modulator at the side of the faulty transmission device, thus ensuring the backup at the time of fault occurrence.

CONSTITUTION: In case the fault occurs at transmission device 1 under transmission of the signals for selection and control from the side of device 1 and thus the transmission becomes impossible for the selection and control signals, the output of modulation part 5 is locked via the software of the microcomputer provided at device 1 or in a hardware way. And then the mark-held signal is transmitted to device 2 from modulator 5. In other words, the signal transmission is broken automatically for modulator 5. Thus the delay is detected at the side of demodulator 6 via the mark-held signal or the mark space signal transmitted to demodulator 6 by the automatic breaking. Then modulator is reset automatically via the software or the hardware of the microcomputer provided at device 1.