PURPOSE: To enable thermal treatment controlled by a correct temperature measurement by substantially equalizing responses of pyrometers on a reference object within a chamber with one reference temperature to accurately determine an emissivity of a semiconductor wafer.

CONSTITUTION: A single wavelength pyrometer 9 detects radiation from an external wall of a quartz processing chamber 13 and a bicolor pyrometer 11 detects radiation from the bottom part or the rear of a semiconductor wafer 15 in a treating chamber 13. The pyrometer 9 is so arranged to have a visual field 16 containing a quartz wall 13 avoiding a lamp 17. The pyrometer 11 is so arranged to have a visual field 18 containing the bottom part or the rear of the wafer 15 avoiding the quartz wall through a visual field opening in the quartz wall having radiative conduction characteristic substantially differing from the lamp 17 and the quartz wall 13. This enables accurately determining of the emissivity of the semiconductor wafer and accurate measurement of temperature thereby achieving an accurate control of thermal treatment for the formation of an acid layer of the wafer.