PURPOSE: To monitor a broken line of an optical fiber by detecting the sensitivity of a 2nd photodetector and the deterioration in the transfer ratio of an optical path up to the 2nd photodetector based on a ratio of an optical intensity of a local oscillating light source to a DC output component of the 2nd photodetector.

CONSTITUTION: The sensitivity of a 2nd photodetector 103 and the deterioration in the transfer ratio of an optical path from a local oscillation light source 101 up to the 2nd photodetector 103 are detected based on a ratio of an optical intensity of the local oscillating light source 101 to a DC output component of the 2nd photodetector 103. When the ratio of an optical intensity of the local oscillating light source 101 to a DC output component of the 2nd photodetector 103 reaches a preset threshold level or below, an alarm is raised. Thus, it is possible to repair a fault before large deterioration takes place in the reception sensitivity and a broken line is monitored.