In a transistor including an oxide semiconductor film, a metal oxide film for preventing electrification which is in contact with the oxide semiconductor film and covers a source electrode and a drain electrode is formed. Then, oxygen is introduced (added) to the oxide semiconductor film through the metal oxide film and heat treatment is performed. Through these steps of oxygen introduction and heat treatment, impurities such as hydrogen, moisture, a hydroxyl group, or hydride are intentionally removed from the oxide semiconductor film, so that the oxide semiconductor film is highly purified. Further, by providing the metal oxide film, generation of a parasitic channel on a back channel side of the oxide semiconductor film can be prevented in the transistor.