A region containing a high proportion of crystal components and a region containing a high proportion of amorphous components are formed separately in one oxide semiconductor film. The region containing a high proportion of crystal components is formed so as to serve as a channel formation region and the other region is formed so as to contain a high proportion of amorphous components. It is preferable that an oxide semiconductor film in which a region containing a high proportion of crystal components and a region containing a high proportion of amorphous components are mixed in a self-aligned manner be formed. To separately form the regions which differ in crystallinity in the oxide semiconductor film, first, an oxide semiconductor film containing a high proportion of crystal components is formed and then process for performing amorphization on part of the oxide semiconductor film is conducted.