It is an object to drive a semiconductor device at high speed or to improve the reliability of the semiconductor device. In a method for manufacturing the semiconductor device, in which a gate electrode is formed over a substrate with an insulating property, a gate insulating film is formed over the gate electrode, and an oxide semiconductor film is formed over the gate insulating film, the gate insulating film is formed by deposition treatment using high-density plasma. Accordingly, dangling bonds in the gate insulating film are reduced and the quality of the interface between the gate insulating film and the oxide semiconductor is improved.