An embodiment of the disclosed invention is a method for manufacturing a semiconductor device, which includes the steps of: forming a first insulating film; performing oxygen doping treatment on the first insulating film to supply oxygen to the first insulating film; forming a source electrode, a drain electrode, and an oxide semiconductor film electrically connected to the source electrode and the drain electrode, over the first insulating film; performing heat treatment on the oxide semiconductor film to remove a hydrogen atom in the oxide semiconductor film; forming a second insulating film over the oxide semiconductor film; and forming a gate electrode in a region overlapping with the oxide semiconductor film, over the second insulating film. The manufacturing method allows the formation of a semiconductor device including an oxide semiconductor, which has stable electrical characteristics and high reliability.