A semiconductor wafer having a surface with a thin film formed thereon is transported into a chamber and held by a holder. After an atmosphere provided in the chamber is replaced, flashes of light are directed from flash lamps in a light irradiation part toward the semiconductor wafer to perform a baking process on the thin film. The irradiation of the semiconductor wafer with light from halogen lamps in the light irradiation part also starts at the same time as the irradiation thereof with the flashes of light. The flashes of light emitted for an extremely short period of time and having a high intensity allow the surface temperature of the thin film to rise momentarily. This prevents the occurrence of abnormal grain growth resulting from prolonged baking in the film.