Abstract: In order to measure the temperature of an object to be processed that has a high infrared transparency in a lamp-heater-equipped chamber filled with an erosive gas, the following measures are taken. A groove is formed in a stage for holding a single silicon wafer substrate attached on the top of a lamp heater or in an enclosure made from quartz and the like provided on a light emitting open section side of the lamp heater so that a thermocouple to be embedded does not contact the erosive gas. To address the issue described above, an equivalent of an object to be processed, namely a small piece of a silicon wafer substrate, is bonded to the thermocouple. Prior to temperature measurement, the difference between a measured temperature of a silicon wafer substrate to be measured placed on the surface of the stage and a measured temperature of the small silicon wafer piece is measured. The difference in thermal capacity between the silicon wafer substrate and the silicon wafer substrate piece is corrected.