PURPOSE: To attain ease of presence or absence of discrimination for a fault of a transmission circuit by providing a phase modulator to modulate a signal to a signal with a shifted phase to a signal of one transmission circuit to other circuit of parallel transmission circuits and detecting an AM component of an output signl of a signal synthesizer.

CONSTITUTION: In a normal state, the signal synthesizer 4 inputs a signal 10 from an A transmission circuit 2 and a signal 11 outputted from a B transmission circuit 3 after being subjected to phase modulation, and its AM component 13 of the output signal is detected b an AM detector 7. The AM component has a frequency component being double the frequency of the modulation signal 12. The AM component detected in this way is inputted to a synchronizing detector 9. On the other hand, the modulation signal 12 is converted into a signal having a double frequency by a two multiplier and inputted to the synchronizing detector 9 as a signal for synchronizing detection. Then it is compared with the said AM component, the presence or absence of the AM component is converted into a DC, and the result is outputted from a terminal 9a. If the transmission of the A transmission circuit 2 is disabled, the relation between the input signal and output signal in the signal synthesizer 4 is as shown in Vector Diagram (b), the envelope 13 of the output signal of the synthesizer 4 has no AM component and then a fault in the transmission circuit is decided.