A manufacturing method of a semiconductor device of the present invention includes the steps of forming a first insulating film over a substrate, forming a semiconductor film over the first insulating film, oxidizing or nitriding the semiconductor film by conducting a plasma treatment to the semiconductor film under a condition of an electron density of 1.times.10.sup.11 cm.sup.-3 or more and 1.times.10.sup.13 cm.sup.-3 or less and an electron temperature of 0.5 eV or more and 1.5 eV or less, using a high frequency wave, forming a second insulating film to cover the semiconductor film, forming a gate electrode over the second insulating film, forming a third insulating film to cover the gate electrode, and forming a conductive film over the third insulating film.