A metal element of a metal film is introduced into the oxide semiconductor film by performing heat treatment in the state where the oxide semiconductor film is in contact with the metal film, so that a low-resistance region having resistance lower than that of a channel formation region is formed. A region of the metal film, which is in contact with the oxide semiconductor film, becomes a metal oxide insulating film by the heat treatment. After that, an unnecessary metal film is removed. Thus, the metal oxide insulating film can be formed over the low-resistance region.