A transistor including an oxide semiconductor layer and having electric characteristics required depending on an intended use, and a semiconductor device including the transistor are provided. In a transistor in which a semiconductor layer, a source electrode layer and a drain electrode layer, a gate insulating film, and a gate electrode layer are stacked in this order over an oxide insulating film, an oxide semiconductor stack composed of at least two oxide semiconductor layers having different energy gaps is used as the semiconductor layer. Oxygen and/or a dopant may be introduced into the oxide semiconductor stack.