A semiconductor device includes a base insulating film including silicon, an oxide semiconductor film over the base insulating film, a gate insulating film over the oxide semiconductor film, a gate electrode which is in contact with the gate insulating film and overlaps with at least the oxide semiconductor film, and a source electrode and a drain electrode electrically connected to the oxide semiconductor film. The oxide semiconductor film includes a region in which a concentration of silicon distributed from the interface with the base insulating film toward an inside of the oxide semiconductor film is lower than or equal to 1.0 at. %. A crystal portion is included at least in the region.