A change in electrical characteristics is suppressed and reliability is improved in a semiconductor device provided with a transistor including an oxide semiconductor. A semiconductor device includes a transistor. The transistor includes a gate electrode, a first insulating film over the gate electrode, a second insulating film over the first insulating film, an oxide semiconductor film over the second insulating film, a first buffer film over the oxide semiconductor film, a second buffer film over the oxide semiconductor film, a source electrode electrically connected with the oxide semiconductor film, and a drain electrode electrically connected with the oxide semiconductor film. The source electrode is electrically connected with the oxide semiconductor film through the first buffer film. The drain electrode is electrically connected with the oxide semiconductor film through the second buffer film.