A semiconductor wafer is heated by a flash of light emitted from a flash lamp after being preheated by a halogen lamp. Temperature of the semiconductor wafer immediately before the flash of light is emitted is measured by a lower radiation thermometer. At the time of irradiation with a flash of light, an upper radiation thermometer measures temperature increase of a front surface of the semiconductor wafer. Front surface temperature of the semiconductor wafer is calculated by adding the temperature increase of the front surface of the semiconductor wafer at the time of irradiation with a flash of light measured by the upper radiation thermometer to the back surface temperature of the semiconductor wafer measured by the lower radiation thermometer.