PURPOSE: To check the leakage of radiation by winding an optical fiber on the perimeter of an equipment generating radiation to detect the transmission loss thereof.

CONSTITUTION: An optical fiber is wound on the perimeter of a nuclear reactor vessel 1 in several turns and optical connectors 3 and 4 are mounted across an optical fiber coil 2. A light emitting element 5 is provided on the connector 3 while a light receiving element 6 near the connector 4. A monitor light receiving element 10 is provided near the element 5. When no radiation is leaked from the vessel 1, the transmission loss of the coil 2 is limited and the output of a detection light receiving element is Id> the output Ia of the monitor light receiving element. When radiation is leaked, the output shifts to Id< the output Ia and a level meter 8 indicates these values to actuate an alarm 9.