To improve the accuracy of calibration, of a radiation thermometer and any other thermometers used in a high temperature range for realizing a fixed point in the temperature range over the copper point.

The thermal fixed point device has a thermal fixed point crucible 4 therein and a furnace in which the ambient temperature of the thermal fixed point crucible 4 is raised and dropped, and the change of temperature is measured using the thermometer, and the thermometer 9 is calibrated based on the state of the change of temperature measured. The thermal fixed point crucible 4 is composed of a crucible, the element of which is carbon, and a fixed point material which is inserted and sealed in the crucible and has the eutectic structure of carbide and carbon or the eutectic structure of carbon solid solution and carbon.