One object is to provide a p-channel transistor including an oxide semiconductor. Another object is to provide a complementary metal oxide semiconductor (CMOS) structure of an n-channel transistor including an oxide semiconductor and a p-channel transistor including an oxide semiconductor. A p-channel transistor including an oxide semiconductor includes a gate electrode layer, a gate insulating layer, an oxide semiconductor layer, and a source and drain electrode layers in contact with the oxide semiconductor layer. When the electron affinity and the band gap of an oxide semiconductor used for the oxide semiconductor layer in the semiconductor device, respectively, are χ (eV) and Eg (eV), the work function (φm) of the conductor used for the source electrode layer and the drain electrode layer satisfies φm>χ+Eg/2 and the barrier for holes (φBp) represented by (χ+Eg−φm) is less than 0.25 eV.