A miniaturized transistor having high electric characteristics is provided with high yield. In a semiconductor device including the transistor, high performance, high reliability, and high productivity are achieved. In a semiconductor device including a transistor in which an oxide semiconductor film, a gate insulating film, and a gate electrode layer on side surfaces of which sidewall insulating layers are provided are stacked in this order, source and drain electrode layers are provided in contact with the oxide semiconductor film and the sidewall insulating layers. In a process for manufacturing the semiconductor device, a conductive film and an interlayer insulating film are stacked to cover the oxide semiconductor film, the sidewall insulating layers, and the gate electrode layer, and the interlayer insulating film and the conductive film over the gate electrode layer are removed by a chemical mechanical polishing method, so that the source and drain electrode layers are formed.