In a top-gate transistor in which an oxide semiconductor film, a gate insulating film, a gate electrode layer, and a silicon nitride film are stacked in this order and the oxide semiconductor film includes a channel formation region, nitrogen is added to regions of part of the oxide semiconductor film and the regions become low-resistance regions by forming a silicon nitride film over and in contact with the oxide semiconductor film. A source and drain electrode layers are in contact with the low-resistance regions. A region of the oxide semiconductor film, which does not contact the silicon nitride film (that is, a region overlapping with the gate insulating film and the gate electrode layer) becomes the channel formation region.