An object is to provide a semiconductor device including an oxide semiconductor with stable electric characteristics can be provided. An insulating layer having many defects typified by dangling bonds is formed over an oxide semiconductor layer with an oxygen-excess mixed region or an oxygen-excess oxide insulating layer interposed therebetween, whereby impurities in the oxide semiconductor layer, such as hydrogen or moisture (a hydrogen atom or a compound including a hydrogen atom such as H2O), are moved through the oxygen-excess mixed region or oxygen-excess oxide insulating layer and diffused into the insulating layer. Thus, the impurity concentration of the oxide semiconductor layer is reduced.