To provide a semiconductor device including an oxide semiconductor which is capable of having stable electric characteristics and achieving high reliability, by a dehydration or dehydrogenation treatment performed on a base insulating layer provided in contact with an oxide semiconductor layer, the water and hydrogen contents of the base insulating layer can be decreased, and by an oxygen doping treatment subsequently performed, oxygen which can be eliminated together with the water and hydrogen is supplied to the base insulating layer. By formation of the oxide semiconductor layer in contact with the base insulating layer whose water and hydrogen contents are decreased and whose oxygen content is increased, oxygen can be supplied to the oxide semiconductor layer while entry of the water and hydrogen into the oxide semiconductor layer is suppressed.