PURPOSE: To measure the temperature of a semiconductor substrate accurately, by projecting infrared rays on the semiconductor substrate, measuring the amount of the first infrared rays, measuring the amount of the second infrared rays without projecting the measuring infrared rays on the semiconductor substrate, and computing the difference between the amount of the infrared rays.

CONSTITUTION: The amount of measuring infrared rays, which are emitted from an emitting means 101, on a semiconductor substrate 103, is controlled by a light-amount controlling means 102. The measuring infrared rays, whose amount is higher than a specified value, are projected. The contributing part of the transmitted infrared rays and the contributing amount of the infrared rays, which are radiated by the temperature of a substrate 103 itself, are overlapped. The amount of the resultant first infrared rays is measured with a measuring means 104. Then the amount of the measuring infrared light on the substrate 103 is made zero, and only the amount of the second infrared rays caused by the temperature of the substrate 103 itself is measured with the measuring means 104. The temperature of the substrate 103 is computed based on the difference between the amounts of the first and second infrared rays in an operating means 105. Thus the temperature of the substrate 103 is accurately measured.