Abstract: A method and an apparatus for measuring a heated material in an industrial furnace by the use of a radiation thermometer comprising providing a sliding means at the tip portion of the radiation thermometer, changing a distance between the heated material and the shielding means to determine background radiation rate of the energy reflected or emitted from the atmosphere of the furnace. The background radiation rate is determined by the output energy of the radiation thermometer, which together with an accurate temperature of the heated material and an ambient temperature of the furnace, are used to calculate a temperature error at arbitrary distances. Setting the shielding means of the radiation thermometer in a position at which the temperature error is within a tolerable range, and measuring the heated material with use of the radiation thermometer.