In producing a semiconductor device such as a thin film transistor (TFT), a silicon semiconductor film is formed on a substrate having an insulating surface, such as a glass substrate, and then a silicon nitride film is formed on the silicon semiconductor film. After that, a hydrogen ion, fluorine ion, or chlorine ion is introduced into the silicon semiconductor film through the silicon nitride film, and then the silicon semiconductor film into which an ion is introduced is heated in an atmosphere containing hydrogen, fluorine, chlorine or these mixture, to neutralize dangling bonds in the silicon semiconductor film and reduce levels in the silicon semiconductor film.