Provided are a transistor which has electrical characteristics requisite for its purpose and uses an oxide semiconductor layer and a semiconductor device including the transistor. In the bottom-gate transistor in which at least a gate electrode layer, a gate insulating film, and the semiconductor layer are stacked in this order, an oxide semiconductor stacked layer including at least two oxide semiconductor layers whose energy gaps are different from each other is used as the semiconductor layer. Oxygen and/or a dopant may be added to the oxide semiconductor stacked layer.