To reduce the adverse affect that characteristics of end portions of a channel forming region of a semiconductor film have on characteristics of a transistor. A gate electrode is formed over a channel forming region of a semiconductor film over a substrate, with a gate insulating film interposed therebetween. The semiconductor film is disposed in a region inside end portions of the gate insulating film. A side surface of the channel forming region is not in contact with at least the gate insulating film, so there is a space enclosed by the substrate, the side surface of the channel forming region, and the gate insulating film. Further, the side surface of the channel forming region is not necessarily in contact with the gate electrode. There may be a space enclosed by the substrate, the side surface of the channel forming region, the gate insulating film, and the gate electrode.