PURPOSE: To simplify the calibration of a temperature signal of a furnace and raise the accuracy of inspection, by detecting the contamination of a window glass through a reference heater and calibrating the temperature signal depending on the result of the detection at the time of measurement of the temperature of the furnace.

CONSTITUTION: When the temperature signal is calibrated, the measurement is suspended and a gate valve 8 is closed. The reference heater 11, whose temperature is cotrolled to be uniformly distributed by a temperature regulator 21, is inserted into a calibration chamber 9. The surface temperature distribution of the reference heater 11 is measured by a temperature detector 7 to produce a signal. The signal is applied together with the temperature signal of the reference heater 11 to a calibration unit 22 so that these signals are compared with each other and the degree of contamination of the window glass 5 is calculated and stored. When the temperature in the furnace is measured, a furnace temperature signal applied to a temperature detector 7 is calibrated on the basis of the stored information.