The electrical characteristics of a transistor including an oxide semiconductor layer are varied by influence of an insulating film in contact with the oxide semiconductor layer, that is, by an interface state between the oxide semiconductor layer and the insulating film. A first oxide semiconductor layer S1, a second oxide semiconductor layer S2, and a third oxide semiconductor layer S3 are sequentially stacked, so that the oxide semiconductor layer through which carriers flow is separated from the gate insulating film containing silicon. The thickness of the first oxide semiconductor layer S1 is preferably smaller than those of the second oxide semiconductor layer S2 and the third oxide semiconductor layer S3, and is less than or equal to 10 nm, preferably less than or equal to 5 nm.