PURPOSE: To detect a break of an optical line, by placing photodetector in advance at a position having possibility of the break of the optical line, and detecting leakage light generated from the optical line, by the photodetector, when the break is generated in its optical line.

CONSTITUTION: Between a base station 1 and a base station of the other side, an optical line 2 is laid as an underground cable, the optical line 2 is led to a manhole 3 at a prescribed interval, and in this place, the connection is executed (a connecting point 4 is shown by a mark X). In the vicinity of the connecting point 4 of the optical line 2, a photodetector 7 is placed, and a detecting output of the detector 7 is connected to a receiving device 9 of the base station 1 by making use of an electric wire 8 for a signal. The photodetector 7 is placed so as to photodetect leakage light when this leakage light is generated from the connecting point 4. Electric power required for operation of the photodetector 7 is supplied through an electric power supplying wire 12 from an electric power source device 11 of the base station 1. By installing a leakage light detector to a connecting point which is apt to generate a break fault most frequently, the break is detected instantaneously, and its position can be searched.