PURPOSE: To make it possible to detect leakage of sodium with a bridge system without using a specific standard cable, by using a sodium system preheating heater as a standard cable and by identifying leakage of sodium by the change of error voltage due to the energizing characteristic of leakage sodium.

CONSTITUTION: It comprises a preheating heater part H constituted as a sheath cable of a nichrome wire and a cable part R for detection formed from a glass wool- covered nichrome wire, etc., and the preheating part H is connected via a switch 6 to a three-phase power source. Further, the preheating part H and the detecting part R are connected through a resistance path comprising resistors 7, 8, a variable resistor 9 for balance adjustment and a conductive path containing a switch 10. Further, between the connecting point of the resistors 7, 8 and one end of the switch 10, a power source 11 is connected and thereby allowing a current to be supplied to the preheating part H and the detecting part R in detection. Thereby, it is possible to detect leakage of sodium with a bridge system and judge precisely even the leakage position.