Stable electrical characteristics and high reliability are provided for a miniaturized semiconductor device including an oxide semiconductor, and the semiconductor device is manufactured. The semiconductor device includes a base insulating layer; an oxide stack which is over the base insulating layer and includes an oxide semiconductor layer; a source electrode layer and a drain electrode layer over the oxide stack; a gate insulating layer over the oxide stack, the source electrode layer, and the drain electrode layer; a gate electrode layer over the gate insulating layer; and an interlayer insulating layer over the gate electrode layer. In the semiconductor device, the defect density in the oxide semiconductor layer is reduced.