In order to manufacture a highly reliable and compact TFT, it is an object of the present invention to provide a method for manufacturing a semiconductor device for forming a gate electrode, a source wiring and a drain wiring with high reliability, and a semiconductor device. In the method for manufacturing a semiconductor device, a semiconductor film is formed over a substrate having an insulated surface, a gate insulating film is formed over the semiconductor film, a gate electrode is formed over the gate insulating film, and a nitride film is formed over the surface of the gate electrode by nitriding the surface of the gate electrode by using high-density plasma.