In case of a disconnection failure of an optical transmission line, a Raman amplifier generates a loss of main and supervisory signal alarms, an optical amplifier generates a loss of main signal alarm, and a supervisory signal transmission and reception unit detects the loss of supervisory signal. On the other hand, in case of a failure generated in an optical fiber, the supervisory signal transmission and reception unit performs APR control. Thus the optical amplifier stops outputting the main signal and continues to output the supervisory signal. The Raman amplifier generates the loss of main signal alarm, and the supervisory signal transmission and reception unit detects the loss of supervisory signal. The monitoring unit identifies failure point according to a combination of an existence of the loss of signal alarm by the Raman and optical amplifiers and the loss of supervisor signal by the supervisory signal transmission and reception unit.