One object of the present invention is to provide a structure of a transistor including an oxide semiconductor in a channel formation region in which the threshold voltage of electric characteristics of the transistor can be positive, which is a so-called normally-off switching element, and a manufacturing method thereof. A second oxide semiconductor layer which has greater electron affinity and a smaller energy gap than a first oxide semiconductor layer is formed over the first oxide semiconductor layer. Further, a third oxide semiconductor layer is formed to cover side surfaces and a top surface of the second oxide semiconductor layer, that is, the third oxide semiconductor layer covers the second oxide semiconductor layer.