PURPOSE: To measure the temperature of a material to be detected accurately by finding the temperature from the amount of radiant heat arriving at a detection part through an optical path member when the object end surface of the optical path member contacts the material to be detected.

CONSTITUTION: The optical path member 6 is a solid optical rod molded of a heat-resistant optical path material, and its outer circumferential surface is covered with a light shield member 10 except both end surfaces. Heat radiation light from the surface of the material to be detected enters the optical member 6 from the object end surface and is projected upon an optical fiber 14 from the projection end surface 12. The heat radiation light guided through the optical fiber 14 is transduced by a transducer 13 into temperature, which is outputted to a temperature display device 8. A contacting state detecting means 9 grasps the point of time when the object end surface 11 of the optical rod 6 contacts the surface of the material to be detected. Then the temperature at this time is regarded as the surface temperature of the material to be detected.