PURPOSE: To automatically detect variation of emissivity and to measure temp. of molten iron with high accuracy by measuring temp. at tip part in a protecting tube submerged into the molten iron with a radiation thermometer and correcting this output with the measured temp. with a consumable type R thermocouple.

CONSTITUTION: The hemisphere-shaped tip part of radiation thermometer composed of a detecting head 5, connecting tube 6 and the protecting tube 7 is submerged into the molten iron 2 flowing down in an iron tapping trough 1. This radiation thermometer detects the temp. at the tip part in the above protecting tube 7 and this is processed with a signal processing circuit 10 to output the temp. signal. This temp. signal is recorded with a recorder 11 and also, the molten iron temp. is calculated with a computer 12 based on the above output and the prescribed correction factor. In the above molten iron temp. continuously measuring instrument, based on the output of the above radiation thermometer and the molten iron temp. measured with the consumable type R thermocouple 13, the above correction factor is corrected and the corrected correction factor is used to execute calculation of the molten iron temp. By this method, regardless of the variation of emissivity of the molten iron 2 and the protecting tube 7, the temp. of molten iron 2 can be measured with high accuracy.