PURPOSE: To enable accurate decision on the presence of a cooling material regardless of the running condition of a nuclear reactor by arranging a heater adapted to generate heat by γ rays and two thermocouples to measure the temperature in the reactor and the temperature of the heater.

CONSTITUTION: An internal heater 2 adapted to generate heat by γ rays is retained with heat insulators 3 and 4 in a package 1 while the first thermocouple 5 for detecting the temperature of the heater 2 is provided while the second thermocouple 6 for detecting the temperature difference between the heater 2 and the package 1. Signal lines of the thermocouples 5 and 6 are connected to temperature measuring devices 8 and 9 through an M1 calbe 7. When there is a cooling material with no change in the temperature of a nuclear reactor, the temperature difference between the heater 2 and the package 1 will be large while there is none of the cooling material, the temperature difference will be small. When the temperature of the nuclear reactor varies, there is little change in the temperature difference. This enables decision on the pressure of the cooling material.