To suppress a change in electrical characteristics and improve reliability in a transistor including an oxide semiconductor film. Provided is a semiconductor device including a transistor including a first gate electrode, a first insulating film over the first gate electrode, a first oxide semiconductor film over the first insulating film, a source electrode electrically connected to the first oxide semiconductor film, a drain electrode electrically connected to the first oxide semiconductor film, a second insulating film over the first oxide semiconductor film, a second oxide semiconductor film as a second gate electrode over the second insulating film, and a third insulating film over the second oxide semiconductor film. The second insulating film includes an excess oxygen region having a concentration gradient.