After a semiconductor wafer held by a susceptor in a chamber is preheated by irradiating the semiconductor wafer with light from halogen lamps, the semiconductor wafer is irradiated with a flash of light from flash lamps. A temperature of the semiconductor wafer is measured with an end edge part radiation thermometer and a temperature of the susceptor is measured with a central part radiation thermometer. It is determined that the semiconductor wafer bows when a specific singular point appears in a temporal differentiation in the temperature measured with the end edge part radiation thermometer or a temporal differentiation in the temperature measured with the central part radiation thermometer. A flash light irradiation of the semiconductor wafer which is determined to be in the bowing state is omitted and the semiconductor wafer is transported out of the chamber, and the subsequent semiconductor wafer is transported into the chamber.