PURPOSE: To measure the temperature in a furnace even at the time of an accident, by switching a constant voltage compensating power supply to a variable compensation power supply even when the temperature in a thermostatic chamber exceeds a prescribed temperature.

CONSTITUTION: When a trouble such as an accident in a reactor is generated, the atmosphere of the inside of a magazine 2 is changed and the temperature in the thermostatic chamber 3 can not be kept at a prescribed cold junction temperature, the inputted resistance value of a temperature measuring resistor 15 exceeds a stored resistance value, so that a comparator 17 outputs a command signal indicating the switching of a relay 13 to connect the variable compensation power supply 16 to an adder 12. Since the variable compensation power supply 16 converts the inputted resistance value of the temperature measuring resistor 15 into the thermoelectromotive force of a reference contact (y) at the temperature in the thermostatic chamber at that time and outputs the converted thermoelectromotive force, the adder 12 outputs voltage equal to the thermoelectromotive force of a measuring contact X, making it possible to measure the temperature in the furnace.