PURPOSE: To enable repetitive use of thermo-couples together with determining correct information on the temperature change of raw materials in blast furnace by suspending a water-cooled protective tube from the blast furnace-top into the raw materials, wherein said thermo-couple is inserted into the protective tube so as to move downwardly according to the movement of the raw materials.

CONSTITUTION: A water cooled protective tube 10 is suspended from the top 2 of the furnace 1 into raw materials 4. A thermocouple 9 is inserted into the tube 9 so that it is reached into raw materials 4a the position of which is below the lower end 12 of the protective tube. The temperature change of raw materials in the furnace can be informed correctively, wthout any influence due to water-cooling, by lowering the protective tube 10 and the thermo-couple 9 in accordance with descending the raw materials during operation of the furnace. The thermo-couple 9 can easily be retrieved after completion of temperature measurement, thus allowing it to be used repetitively without replacing new one for every measurement as in conventional methods.