In a process of manufacturing a transistor including an oxide semiconductor layer, an amorphous oxide semiconductor layer which includes a region containing excess oxygen as compared to a stoichiometric composition ratio of an oxide semiconductor in a crystalline state is formed over a silicon oxide film, an aluminum oxide film is formed over the amorphous oxide semiconductor layer, and then heat treatment is performed so that at least part of the amorphous oxide semiconductor layer is crystallized and an oxide semiconductor layer which includes a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer is formed.