To perform continuous temperature measurement while correcting the error in a radiation thermometer in on-line caused by the progressing of contamination on an observing window, etc., in a method for performing the radiation temperature measurement of molten steel through a nozzle in a refractory vessel wall.

The molten steel temperature is continuously measured with the radiation thermometer fitted to the observing nozzle at the furnace bottom in the refractory vessel of a molten steel refining furnace. A thermocouple is intermittently dipped into the molten steel from the opening part at the upper part of the refractory vessel and the temperature measurement is instantaneously performed. Just after performing the temperature measurement with the thermocouple, the temperature with the thermocouple is used as true temperature and the variation of transmissivity in the observing window is grasped. The continuous temperature measurement is performed while correcting the indicated temperature of the radiation thermometer with the newest transmissivity value.