As a transistor including an oxide semiconductor film, a transistor in which a metal oxide film containing a constituent similar to that of an oxide semiconductor film is provided between the oxide semiconductor film and a gate insulating film and a gate insulating film containing a constituent different from that of the metal oxide film and that of the oxide semiconductor film is provided to be in contact with the metal oxide film is provided. The oxide semiconductor film used for an active layer of the transistor is a highly purified and electrically i-type (intrinsic) film which is formed by heat treatment through which an impurity such as hydrogen, moisture, a hydroxyl group or a hydride is removed and oxygen which is a main component of the oxide semiconductor and reduced together with the impurity removal step is supplied.