A semiconductor wafer is preheated with a halogen lamp, and then is heated by irradiation with a flash of light emitted from a flash lamp. The preheating with the halogen lamp is continued for a short time even after the flash lamp turns off. A radiation thermometer measures a front surface temperature and a back surface temperature of the semiconductor wafer. A temperature integrated value is calculated by integration of temperatures of the semiconductor wafer measured during a period from a start of the flash irradiation to an end of the heating of the semiconductor wafer. It is determined that the semiconductor wafer is cracked at the time of flash irradiation when the calculated temperature integrated value deviates from a preset range between an upper limit value and a lower limit value.