PURPOSE: To enable inexpensive fault detection which uses no multicore cable by detecting an in-tunnel illuminating lamp and equipment fault by a photodetector and a penetration type CT and estimating the fault position through a single-core optical fiber cable.

CONSTITUTION: The light of the illuminating lamp 1 is detected by the photodetector 2, whose output is inputted to an electromagnetic actuator 3 and its operating lever 3a abuts on one optical fiber in a tunnel and is pressed against the recessed part 4a of a cradle 4 to form a locally curved part. Therefore, if the illuminating lamp 1 becomes faulty not to emit the light, the actuator 3 operates to form a small curved part of the optical fiber 5. Then transmission loss is generated and detected by an optical fiber analyzer 6. Further, the small curved part is formed and then a step is generated at the position, so the analyzer 6 detects the position. A through type current transformer CT is mounted on the power line, etc., of an instrument or monitor equipment other than the illuminating lamp and its output is inputted to the electromagnetic actuator 3, which can operate to detect an abnormal place.