PURPOSE: To measure true temperature without the effect of the emissivity of a body to be measured, by moving a reflecting mirror having a through hole to two far and near positions toward the body to be measured.

CONSTITUTION: A reflecting mirror 3 having a through hole 3a at the center is provided between a body to be measured and a radiation thermometer 2. Energy radiated from the body to be measured is reflected multiple times by the reflecting mirror 3. The radiation energy passing the through hole 3a of the reflecting mirror 3 is measured by the radiation thermometer 2. The reflecting mirror 3 is moved to two far and near positions from the body to be measured within the field of view of the radiation thermometer. The temperature of the body to be measured is obtained by using the specified expression based on the following parameters: the measured energy, which is inputted to the radiation thermometer 2 from the through hole 3a of the reflecting mirror 3; the measured value of the radiation energy of the body to be measured itself when the reflecting mirror 3 is evacuated out of the field of view of the ratiation thermometer; and the arresting ratio of the components of the multiple reflections in the radiation thermometer 2.