An object of an embodiment of the present invention is to provide a semiconductor device provided with a thin film transistor which includes an oxide semiconductor layer and has high electric characteristics. The semiconductor device includes a gate electrode over an insulating surface, an oxide semiconductor layer including silicon oxide, an insulating layer between the gate electrode and the oxide semiconductor layer, and source and drain regions between the oxide semiconductor layer including silicon oxide and source and drain electrode layers. The source and drain regions are formed using a degenerate oxide semiconductor material or a degenerate oxynitride material.