An object of the present invention is to provide a method for manufacturing a semiconductor device with high reliability, at low cost, in which an element forming layer having a thin film transistor and the like provided over a substrate is peeled from the substrate, so that a semiconductor device is manufactured. According to the invention, a metal film is formed over a substrate, a plasma treatment is performed to the metal film in a dinitrogen monoxide atmosphere to form a metal oxide film over the metal film, a first insulating film is formed continuously without being exposed to the air, an element forming layer is formed over the first insulating film, and the element forming layer is peeled from the substrate, so that a semiconductor device is manufactured.