A semiconductor device with significantly low off-state current is provided. An oxide semiconductor material in which holes have a larger effective mass than electrons is used. A transistor is provided which includes a gate electrode layer, a gate insulating layer, an oxide semiconductor layer including a hole whose effective mass is 5 or more times, preferably 10 or more times, further preferably 20 or more times that of an electron in the oxide semiconductor layer, a source electrode layer in contact with the oxide semiconductor layer, and a drain electrode layer in contact with the oxide semiconductor layer.