An object of an embodiment of the present invention is to manufacture a highly-reliable semiconductor device comprising a transistor including an oxide semiconductor, in which change of electrical characteristics is small. In the transistor including an oxide semiconductor, oxygen-excess silicon oxide (SiOX (X>2)) is used for a base insulating layer of a top-gate structure or for a protective insulating layer of a bottom-gate structure. By using the oxygen-excess silicon oxide, oxygen is discharged from the insulating layer, and oxygen deficiency of an oxide semiconductor layer and the interface state density between the oxide semiconductor layer and the base insulating layer or the protective insulating layer can be reduced, so that the highly-reliable semiconductor device in which change of electrical characteristics is small can be manufactured.