A highly reliable semiconductor device having stable electrical characteristics is provided. Oxide films each containing one or more kinds of metal elements included in an oxide semiconductor film are formed in contact with an upper side and a lower side of the oxide semiconductor film where a channel is formed, whereby interface states are not easily generated at an upper interface and a lower interface of the oxide semiconductor film. A material which has a lower electron affinity than the oxide semiconductor film is used for the oxide films in contact with the oxide semiconductor film, whereby electrons flowing in the channel hardly move in the oxide films and mainly move in the oxide semiconductor film. Thus, even when an interface state exists between the oxide film and an insulating film formed on the outside of the oxide film, the state hardly influences the movement of electrons.