A method for manufacturing a semiconductor device comprising the steps of: forming a first oxide semiconductor film over a substrate having an insulating surface; forming a second oxide semiconductor film over the first oxide semiconductor film; heating the first oxide semiconductor film and the second oxide semiconductor film; after the heating, cooling the first oxide semiconductor film and the second oxide semiconductor film in an atmosphere containing oxygen; selectively etching the first oxide semiconductor film and the second oxide semiconductor film after the cooling to form a first oxide semiconductor layer from the first oxide semiconductor film and a second oxide semiconductor layer from the second oxide semiconductor film; forming a conductive film over the first oxide semiconductor layer and the second oxide semiconductor layer; selectively etching the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive film to form from the first oxide semiconductor layer a semiconductor layer including a channel formation region, from the second oxide semiconductor layer a source region and a drain region (404a; 404b), and from the conductive film a source electrode layer and a drain electrode layer (405a; 405b); and forming an oxide insulating film which is in contact with part of the semiconductor layer, the source region, the drain region (404a; 404b), the source electrode layer, and the drain electrode layer (405a; 405b); wherein heating is performed for dehydration or dehydrogenation of the first oxide semiconductor film and the second oxide semiconductor film; wherein cooling is performed at a temperature higher than or equal to room temperature and lower than 100 °C; and wherein by forming the oxide insulating film in contact with part of the semiconductor layer which includes a channel formation region, the carrier density of the semiconductor layer is reduced. forming a first oxide semiconductor film over a substrate having an insulating surface; forming a second oxide semiconductor film over the first oxide semiconductor film; heating the first oxide semiconductor film and the second oxide semiconductor film; after the heating, cooling the first oxide semiconductor film and the second oxide semiconductor film in an atmosphere containing oxygen; selectively etching the first oxide semiconductor film and the second oxide semiconductor film after the cooling to form a first oxide semiconductor layer from the first oxide semiconductor film and a second oxide semiconductor layer from the second oxide semiconductor film; forming a conductive film over the first oxide semiconductor layer and the second oxide semiconductor layer; selectively etching the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive film to form from the first oxide semiconductor layer a semiconductor layer including a channel formation region, from the second oxide semiconductor layer a source region and a drain region (404a; 404b), and from the conductive film a source electrode layer and a drain electrode layer (405a; 405b); and forming an oxide insulating film which is in contact with part of the semiconductor layer, the source region, the drain region (404a; 404b), the source electrode layer, and the drain electrode layer (405a; 405b); wherein heating is performed for dehydration or dehydrogenation of the first oxide semiconductor film and the second oxide semiconductor film; wherein cooling is performed at a temperature higher than or equal to room temperature and lower than 100 °C; and wherein by forming the oxide insulating film in contact with part of the semiconductor layer which includes a channel formation region, the carrier density of the semiconductor layer is reduced.