PURPOSE: To measure the temperature of the body accurately by measuring the temperature of atmosphere in a furnace whose temperature is uniform by a cavity radiation, and compensating the measured temperature of the object.

CONSTITUTION: The object 2 is arranged in a heating furnace 1. Holes 31 and 32 are provided in the wall of the heating furnace 1. A radiation thermometer which measures the temperature of the objects 2 is provided in the hole 31. A thermoelectric thermometer 5 which is provided in a protecting tube and measures the temperature of the atmosphere in the furnace is protruded from the hole 32. The temperature signal of the thermoelectric thermometer 5 is inputted to a transducer 6, and converted into a signal corresponding to the radiation. A compensator 7 outputs the true temperature of the object 2 from the output signal of the radiation thermometer 4 and the output signal of the transducer 6, and said output is displayed on a display 8.