To precisely measure the temperature of a substrate under treatment without being infulenced by a noise such as a high frequency for generating a plasma by providing a dimension measuring means for measuring the dimension of a specified part of a substrate, and a calculating means for calculating the temperature of the substrate on the basis of the measurement result.

A laser beam is emitted from a light emitting part 2a into a treatment chamber A through a window B1 to irradiate a substrate 10, the image of the shadow of the substrate is gained through a window B2, and delivered to a calculating part 3. In the calculating part 3, the relation between the temperature and the outer dimension of the substrate 10 is preliminarily prepared from the relation of the thermal expansion by the heat of the substrate 10 to be measured. The calculating part 3 utiizes this to measure the diameter of the substrate from a signal showing the substrate 10 from a light receiving part 2b, and determines the temperature corresponding to the diameter. Thus, even during the treatment of the substrate 10 within the treatment chamber A, the temperature of the substrate 10 can be measured.