One object is to provide a semiconductor device including an oxide semiconductor, which has stable electric characteristics and high reliability. Another object is to manufacture a highly reliable semiconductor device in a high yield. In a top-gate staggered transistor including an oxide semiconductor film, as a first gate insulating film in contact with the oxide semiconductor film, a silicon oxide film is formed by a plasma CVD method with use of a deposition gas containing silicon fluoride and oxygen; and as a second gate insulating film stacked over the first gate insulating film, a silicon oxide film is formed by a plasma CVD method with use of a deposition gas containing silicon hydride and oxygen.