A miniaturized transistor having high electrical characteristics can be provided with high yield. High performance, high reliability, and high productivity of a semiconductor device including the transistor can be achieved. The semiconductor device includes a gate electrode over an insulating surface; a base insulating film which is over the insulating surface and from which the gate electrode protrudes; a gate insulating film over the base insulating film and the gate electrode; an oxide semiconductor film over the gate insulating film; and a source electrode and a drain electrode in contact with an oxide semiconductor film. The thickness of the oxide semiconductor film is smaller than the difference between the thickness of the gate electrode and the thickness of the base insulating film.