A highly reliable semiconductor device including a transistor using an oxide semiconductor is provided. In a semiconductor device including a bottom-gate transistor including an oxide semiconductor layer, a first insulating layer is formed in contact with the oxide semiconductor layer, and an oxygen doping treatment is performed thereon, whereby the first insulating layer is made to contain oxygen in excess of the stoichiometric composition. The formation of the second insulating layer over the first insulating layer enables excess oxygen included in the first insulating layer to be supplied efficiently to the oxide semiconductor layer. Accordingly, the highly reliable semiconductor device with stable electric characteristics can be provided.