A semiconductor device is provided with a first oxide semiconductor film over an insulating surface; a second oxide semiconductor film over the first oxide semiconductor film; a third oxide semiconductor film in contact with a top surface of the insulating surface, a side surface of the first oxide semiconductor film, and side and top surfaces of the second oxide semiconductor film; a gate insulating film over the third oxide semiconductor film; and a gate electrode in contact with the gate insulating film and faces the top and side surfaces a of the second oxide semiconductor film. A thickness of the first oxide semiconductor film is larger than a sum of a thickness of the third oxide semiconductor film and a thickness of the gate insulating film, and the difference is larger than or equal to 20 nm.