A semiconductor device including a thin film transistor which includes an oxide semiconductor layer and has high electric characteristics and reliability. Film deposition is performed using an oxide semiconductor target containing an insulator (an insulating oxide, an insulating nitride, silicon oxynitride, aluminum oxynitride, or the like), typically SiO2, so that the semiconductor device in which the Si-element concentration in the thickness direction of the oxide semiconductor layer has a gradient which increases in accordance with an increase in a distance from a gate electrode is realized.