A substrate in which a high-dielectric-constant gate insulator is formed on a silicon substrate with an interface layer film sandwiched in between is housed in a chamber. The method of the invention including: (a) housing the substrate in a chamber; (b) supplying ammonia to the chamber to foam an ammonia atmosphere; and (c) applying flash light to a surface of the substrate housed in the chamber to heat the high dielectric constant film, wherein the flash light applied in said step (c) has a spectral distribution that has a peak in a wavelength range of 200 to 300 nm.