Light is applied for preheating from a halogen lamp to a lower surface of a semiconductor wafer supported on a susceptor within a chamber. Thereafter, flash light is applied for flash heating from a flash lamp to an upper surface of the semiconductor wafer. High-temperature treatment gas heated by a heater is supplied into the chamber to preheat a structure inside the chamber including a susceptor before heat treatment for an initial semiconductor wafer of a lot starts. By raising the temperature of the structure inside the chamber to a temperature substantially equivalent to a temperature of the structure during steady treatment, all semiconductor wafers constituting the lot are supportable on the susceptor maintained at a constant temperature without the necessity of dummy running. Accordingly, a temperature history is equalized for all the semiconductor wafers.