An insulating layer which releases a large amount of oxygen is used as an insulating layer in contact with a channel region of an oxide semiconductor layer, and an insulating layer which releases a small amount of oxygen is used as an insulating layer in contact with a source region and a drain region of the oxide semiconductor layer. By releasing oxygen from the insulating layer which releases a large amount of oxygen, oxygen deficiency in the channel region and an interface state density between the insulating layer and the channel region can be reduced, so that a highly reliable semiconductor device having small variation in electrical characteristics can be manufactured. The source region and the drain region are provided in contact with the insulating layer which releases a small amount of oxygen, thereby suppressing the increase of the resistance of the source region and the drain region.