PURPOSE: To ensure the detection of faulure, by detecting that the output of transmission unit of reception unit is contiunously 1 or 0 through the input to the transmission unit of the signal transmission unit made continuously 1 or 0 at power supply interruption and other failure.

CONSTITUTION: Normally, the input signal S1 is converted A into required data, is inputted to the unit B through the selector, and after modulation, it is transmitted via the interface L1. The data is demodulated at the signal transmission unit C via the interface L2 of the receiver M and converted E into required signal S2. In case of transmission line open, "0" is continuously inputted to the signal tranmission unit B and the output of the unit C is a continuity of "0", and it is detected at the unit F to produce failure signal S3. At the interruption of power supply, the selector is operated, the battery P in inserted, the input to the unit B is continuously "1" and the output of the unit C is also continuity of "1". The continuous "1" signal is detected at the power supply interruption G to produce the power supply interruption signal S4. Thus, the failure in transmission side can monitored at remote location.