It is an object to form a buffer circuit, an inverter circuit, or the like using only n-channel TFTs including an oxide semiconductor layer. A buffer circuit, an inverter circuit, or the like is formed by combination of a first transistor in which a source electrode and a drain electrode each overlap with a gate electrode and a second transistor in which a source electrode overlaps with a gate electrode and a drain electrode does not overlap with the gate electrode. Since the second transistor has such a structure, the capacitance Cp can be small, and VA′ can be large even in the case where the potential difference VDD−VSS is small.