A flash heating part in a heat treatment apparatus includes 30 built-in flash lamps, and irradiates a semiconductor wafer held by a holder in a chamber with a flash of light. Thirty switching elements are provided in a one-to-one correspondence with the 30 flash lamps. Each of the switching elements defines the waveform of current flowing through a corresponding one of the flash lamps by intermittently supplying electrical charge thereto. Radiation thermometers measure an in-plane temperature distribution of the semiconductor wafer during flash irradiation. Based on the results of measurement with the radiation thermometers, a controller individually controls the operations of the 30 switching elements to individually define the light emission patterns of the 30 flash lamps.