According to one embodiment, a method for manufacturing a semiconductor device, includes forming a first insulating film which covers a gate electrode, forming an island-shaped oxide semiconductor in contact with the first insulating film, forming a second insulating film which covers the oxide semiconductor, forming a source electrode in contact with the oxide semiconductor, forming a drain electrode in contact with the oxide semiconductor, forming a third insulating film which covers the source electrode and the drain electrode and forming a channel region between the source electrode and the drain electrode by supplying oxygen from the third insulating film to the oxide semiconductor via the second insulating film.