The invention relates to a semiconductor device including an oxide semiconductor layer, a gate electrode overlapping with a channel formation region of the oxide semiconductor layer, and a source electrode or a drain electrode overlapping with a first region of the oxide semiconductor layer, and a second region between the channel formation region and the first region. An upper layer of the second region includes a microvoid. The microvoid is formed by adding nitrogen to the upper layer of the second region. Thus, upper layer of the second region contains lager amount of nitrogen than a lower layer of the second region.