A semiconductor device includes an oxide semiconductor layer including a channel formation region which includes an oxide semiconductor having a wide band gap and a carrier concentration which is as low as possible, and a source electrode and a drain electrode which include an oxide conductor containing hydrogen and oxygen vacancy, and a barrier layer which prevents diffusion of hydrogen and oxygen between an oxide conductive layer and the oxide semiconductor layer. The oxide conductive layer and the oxide semiconductor layer are electrically connected to each other through the barrier layer.