The present invention relates to a method for manufacturing a display device including a p-channel thin film transistor and an n-channel thin film transistor having a microcrystalline semiconductor film each of which are an inverted-staggered type, and relates to a method for formation of an insulating film and a semiconductor film which are included in the thin film transistor. Two or more kinds of high-frequency powers having different frequencies are supplied to an electrode for generating glow discharge plasma in a reaction chamber. High-frequency powers having different frequencies are supplied to generate glow discharge plasma, so that a thin film of a semiconductor or an insulator is formed. High-frequency powers having different frequencies (different wavelength) are superimposed and applied to the electrode of a plasma CVD apparatus, so that densification and uniformity of plasma for preventing the effect of surface standing wave of plasma can be realized.