To increase the on-state current of a transistor whose channel is formed in an oxide semiconductor layer. To provide a transistor where a resistance-reducing element is introduced into a region of an oxide semiconductor layer which overlaps with part of a source or drain or part of a gate. For example, the thickness of a region of a conductive layer serving as a source or drain or a gate (at least part of a region overlapping with an oxide semiconductor layer) is made smaller than that of the other region of the conductive layer. A resistance-reducing element is introduced into the oxide semiconductor layer through the conductive layer thinned partly, thereby obtaining the oxide semiconductor layer where the resistance-reducing element is introduced into the region overlapping with part of the source or drain or part of the gate. Thus, the on-state current of the transistor can be increased.