An object is to provide a semiconductor device including an oxide semiconductor, which has stable electric characteristics and high reliability. A semiconductor device includes an oxide semiconductor film, a source electrode and a drain electrode which are electrically connected to the oxide semiconductor film, a metal oxide film which is partly in contact with the oxide semiconductor film, a gate insulating film which is over and in contact with the metal oxide film, and a gate electrode over the gate insulating film. With such a structure, effect of charge on the oxide semiconductor film can be relaxed; thus, shift of the threshold voltage in the transistor, due to charge trapping at an interface of the oxide semiconductor film, can be suppressed.