To provide a semiconductor device including a thin film transistor having excellent electric characteristics and high reliability and a manufacturing method of the semiconductor device with high mass productivity. The summary is that an inverted-staggered (bottom-gate) thin film transistor is included in which an oxide semiconductor film containing In, Ga, and Zn is used as a semiconductor layer, a channel protective layer is provided in a region that overlaps a channel formation region of the semiconductor layer, and a buffer layer is provided between the semiconductor layer and source and drain electrodes. An ohmic contact is formed by intentionally providing the buffer layer having a higher carrier concentration than the semiconductor layer between the semiconductor layer and the source and drain electrodes.