To accurately measure the temperature of a substrate in a treatment chamber by sampling light from the treatment chamber by means of the input terminal section of a probe and connecting the output terminal section of the probe to the main line of a split fiber optical guide.

A reflecting plate 20 is positioned below a substrate 14 in a treatment chamber 12 so that the substrate 14 can be heated quickly and evenly to a high temperature. In a self-calibrating probe 40, one end section of a photoconductor 42 is extended to the top of the reflecting plate 20, makes light incident to the chamber 12, and samples light from the chamber 12. The other end section of the photoconductor 42 is coupled of the light source branch 50 of the main line of a split fiber optical guide 46 and a sampling branch 52 is coupled with a pyrometer 56. The photoconductor 42 samples the light emitted from the substrate 14 and the pyrometer 56 measures the temperature of the substrate 14 based on the luminous intensity of the sampled light, spectral emissivity of the substrate 14, etc. When an object having known reflectivity is housed in the chamber 12 and the probe 40 calibrates itself by measuring 56 the intensity of reflected light from the object, the temperature of the substrate 14 can be measured accurately.