PURPOSE: To obtain an accurate continuous measured value, by correcting a continuous measured value due to a radiation thermometer by using a discontinuous measured value due to a consumption type thermocouple.

CONSTITUTION: A heat resistant pipe 13 is immersed in a molten metal 2. In the next step, after the lid 17 attached to the heat resistant pipe 13 is lost by melting, the temp. of the molten metal 2 is continuously measured by a radiation thermometer 11 through the cavity 14 of the heat resistant pipe 13. This continuous measured value is inputted to a correction operator 9 through a signal converting apparatus 12. Subsequently, a consumption type thermocouple 6 is immersed in the molten metal 2 by an insert apparatus 7 and a discontinuous measured value due to said thermocouple 16 is inputted to a correction operator 9 through a signal converting apparatus 8. The correction operator 9 performs the correcting operation of the measured values to continuously output the temp. of the molten metal to an indication recording apparatus 10.