In a semiconductor device including a transistor using an oxide semiconductor film, stable electric characteristics can be provided and high reliability can be achieved. A structure of the semiconductor device, which achieves high-speed response and high-speed operation, is provided. In a semiconductor device including a transistor in which an oxide semiconductor film, a gate insulating film, and a gate electrode layer are stacked in order and a sidewall insulating layer is provided on the side surface of the gate electrode layer, the sidewall insulating layer has an oxygen-excess regions, which is formed in such a manner that a first insulating film is formed and then is subjected to oxygen doping treatment, a second insulating is formed over the first insulating film, and a stacked layer of the first insulating film and the second insulating film are etched.