PURPOSE: To identify whether an optical fiber is normal or faulty by assigning an identification bit identifying a sender to an idle bit in a frame in addition to a synchronizing signal.

CONSTITUTION: A frame synchronizing signal insertion circuit 1 inserts a specified frame synchronizing signal to a transmission PCM signal 10 and uses an identification bit insertion means to insert a predetermined identification bit (for example, a bit string using '1' as its 1st bit). An identification bit detection circuit 6 detects the identification bit from an output signal of a frame synchronizing circuit 5 subject to synchronization lock to identify and discriminate whether or not an output signal is an adequate reception signal. Thus, if the optical fiber is broken, the transmission signal is reflected in the broken face to cause a reception signal, then misrecognizing the signal as a normal reception signal is prevented.