A transistor which is formed using an oxide semiconductor layer and has electric characteristics needed for the intended use, and a semiconductor device including the transistor are provided. The transistor is formed using an oxide semiconductor stack including at least a first oxide semiconductor layer in contact with a source electrode layer and a drain electrode layer and a second oxide semiconductor layer which is provided over the first oxide semiconductor layer and has an energy gap different from that of the first oxide semiconductor layer. There is no limitation on the stacking order of the first oxide semiconductor layer and the second oxide semiconductor layer as long as their energy gaps are different from each other.