An object is to provide a transistor in which the state of an interface between an oxide semiconductor layer and an insulating film (gate insulating layer) in contact with the oxide semiconductor layer is favorable; and a method for manufacturing the transistor. In order to obtain the transistor, nitrogen is added to a region of the oxide semiconductor layer in the vicinity of the interface with the gate insulating layer. Specifically, a concentration gradient of nitrogen is formed in the oxide semiconductor layer, and a region containing much nitrogen is provided at the interface with the gate insulating layer. By the addition of nitrogen, a region with high crystallinity can be formed in the region of the oxide semiconductor layer in the vicinity of the interface with the gate insulating layer, so that a stable interface state can be obtained.