An optical repeater monitoring system, according to the invention, comprises an oscillating source, a reference signal transmitter for transmitting a reference signal of a predetermined frequency generated from an output of the oscillating source to a first optical fiber, and an optical repeater. The optical repeater has a first photodetector for converting light from the first optical fiber into an electrical signal, a reference signal extractor for extracting a component of the reference signal from an output of the first photodetector, a carrier generator for generating a carrier from an output of the reference signal extractor, a monitor signal modulator for modulating the carrier generated by the carrier generator with a monitor signal showing a operating state of the optical repeater, a transmitter for transmitting an output of the monitor signal modulator to a second optical fiber. The system further comprises a demodulating signal generator for generating a demodulating signal from either of the output from the oscillating source and the reference signal, the demodulating signal having a frequency equal to that of the monitor signal carrier, a second photodetector for photodetecting the light propagated on the second fiber, and a monitor signal demodulator for demodulating the monitor signal from outputs of the second photodetector and demodulating signal generator.