A method of monitoring the wear of refractor walls of a blast furnace wherein temperature is sensed at different points across the thickness of the furnace walls and various electric representations of the internal phenomena of the blast furnace are derived as trigger signals. Analysis is carried out as to the correlation between the trigger signals and variations in the temperatures at the furnace walls in terms of the time delay, thus predicting the present status of the wear of the furnace walls. A temperature probe assembly is also disclosed which is useful in sensing the temperature distribution of the refractory walls. The temperature probe assembly includes a plurality of parallel sheath type thermocouples or thermometers.