In a miniaturized transistor, a gate insulating layer is required to reduce its thickness; however, in the case where the gate insulating layer is a single layer of a silicon oxide film, a physical limit on thinning of the gate insulating layer might occur due to an increase in tunneling current, i.e. gate leakage current. With the use of a high-k film whose relative permittivity is higher than or equal to 10 is used for the gate insulating layer, gate leakage current of the miniaturized transistor is reduced. With the use of the high-k film as a first insulating layer whose relative permittivity is higher than that of a second insulating layer in contact with an oxide semiconductor layer, the thickness of the gate insulating layer can be thinner than a thickness of a gate insulating layer considered in terms of a silicon oxide film.