The present invention intends to improve the accuracy of temperature measurement when measuring the temperature of a semiconductor wafer by a radiation thermometer on the basis of the idea of virtual blackbody simulated by multiple reflection of light. A system includes a wafer (W), a circular reflector 1 of a radius R disposed opposite to the wafer (W), and a probe (2) disposed in a through hole formed in the reflector (1). The probe (2) is a through hole. The radiation intensity of radiation passed the through hole is determined by image data provided by a CCD camera disposed behind the back surface of the reflector (1). An error in measured radiation intensity of radiation falling the probe (2) due to light that enters a space between the wafer (W) and the reflector (1) and a space between the reflector (1) and the probe (2) and light leaks from the same spaces is corrected, the emissivity of the wafer (W) is calculated and the temperature of the wafer (W) is determined.