On a transparent substrate where a gate electrode is formed, an amorphous silicon film is deposited by plasma CVD with a gate insulating film interposed therebetween. The silicon film is heated in an nitrogen atmosphere at 430±20° C. for an hour or longer to discharge hydrogen remaining in the film when it is formed. The silicon film is then melted by laser irradiation to crystallize, to thereby form a polycrystalline silicon film serving as an active region. Thus, when amorphous silicon is crystallized to form a polycrystalline silicon film, it is made possible to prevent creation of a rough film surface and penetration of impurity ions in the atmosphere into the polycrystalline silicon.