A separation layer is formed over a substrate, an insulating film 107 is formed over the separation layer, a bottom gate insulating film 103 is formed over the insulating film 107, an amorphous semiconductor film is formed over the bottom gate insulating film 103, the amorphous semiconductor film is crystallized to form a crystalline semiconductor film over the bottom gate insulating film 103, a top gate insulating film 105 is formed over the crystalline semiconductor film, top gate electrodes 106a and 106b are formed over the top gate insulating film 105, the separation layer is separated from the insulating film 107, the insulating film 107 is processed to expose the bottom gate insulating film 103, and bottom gate electrodes 115a and 115b in contact with exposed the gate insulating film 103 are formed.