It is an object to provide a semiconductor device including a thin film transistor with favorable electric properties and high reliability, and a method for manufacturing the semiconductor device with high productivity. In an inverted staggered (bottom gate) thin film transistor, an oxide semiconductor film containing In, Ga, and Zn is used as a semiconductor layer, and a buffer layer formed using a metal oxide layer is provided between the semiconductor layer and a source and drain electrode layers. The metal oxide layer is intentionally provided as the buffer layer between the semiconductor layer and the source and drain electrode layers, whereby ohmic contact is obtained.