A gate insulating film having an insulating film that contains at least nitrogen is formed on a substrate, and the gate insulating film is subjected to heat treatment for about 500 milliseconds or less using a flash lamp. Thereafter, a gate electrode is formed on the gate insulating film. Specifically, for example, a laminated film of SiO.sub.2 film and an Si.sub.xN.sub.(1-x) film, a laminated film of an SiO.sub.2 film, HfSiO film, and an Si.sub.xN.sub.(1-x) film, or the like, is formed in forming the gate insulating film.