One object is to provide a semiconductor device including an oxide semiconductor with improved electrical characteristics. The semiconductor device includes a first insulating film including an element of Group 13 and oxygen; an oxide semiconductor film partly in contact with the first insulating film; a source electrode and a drain electrode electrically connected to the oxide semiconductor film; a gate electrode overlapping with the oxide semiconductor film; and a second insulating film partly in contact with the oxide semiconductor film, between the oxide semiconductor film and the gate electrode. Further, the first insulating film including an element of Group 13 and oxygen includes a region where an amount of oxygen is greater than that in a stoichiometric composition ratio.