PURPOSE: To detect a sign of abnormality in advance by detecting the abnormality in such a way that a light passing path is formed through a conductor supporting insulator, an optical fiber temperature sensor is provided, and the temperature of a circuit conductor is continuously measured.

CONSTITUTION: After a through hole 4 is formed in the central part of an insulator 3 supporting a conductor and a hole 5 is formed through a flush fitting 6 connected with a main circuit conductor 1, an optical fiber thermometer is inserted into the holes 4 and 5. A detecting element 9 is fitted to the front end of the optical fiber 8 of the thermometer 7 and the element 9 is brought into close contact with the conductor 1 by means of a spring 14. The output of the thermometer 7 is inputted to a signal processing circuit which is not shown in the figure through the fiber 8 and converted into the temperature of the conductor 1. Therefore, the temperature of the main circuit conductor 1 can directly and continuously be measured and the abnormal state of the insulator 3 can be detected.