At this time, the oxide 530b serves as a main carrier path. When the oxide 530a and the oxide 530c have the above structures, the densities of defect states at the interface between the oxide 530a and the oxide 530b and the interface between the oxide 530b and the oxide 530c can be made low. Thus, the influence of interface scattering on carrier conduction is small, and the transistor 500 can have a high on-state current.