A decrease in on-state current in a semiconductor device including an oxide semiconductor film is suppressed. A transistor including an oxide semiconductor film, an insulating film which includes oxygen and silicon, a gate electrode adjacent to the oxide semiconductor film, the oxide semiconductor film provided to be in contact with the insulating film and overlap with at least the gate electrode, and a source electrode and a drain electrode electrically connected to the oxide semiconductor film. In the oxide semiconductor film, a first region which is provided to be in contact with the interface with the insulating film and have a thickness less than or equal to 5 nm has a silicon concentration lower than or equal to 1.0 at. %, and a region in the oxide semiconductor film other than the first region has lower silicon concentration than the first region.