To provide a semiconductor device including an oxide semiconductor layer with high and stable electrical characteristics, the semiconductor device is manufactured by forming a first insulating layer, forming oxide over the first insulating layer and then removing the oxide n times (n is a natural number), forming an oxide semiconductor layer over the first insulating layer, forming a second insulating layer over the oxide semiconductor layer, and forming a conductive layer over the second insulating layer. Alternatively, the semiconductor device is manufactured by forming the oxide semiconductor layer over the first insulating layer, forming the second insulating layer over the oxide semiconductor layer, forming the oxide over the second insulating layer and then removing the oxide n times (n is a natural number), and forming the conductive layer over the second insulating layer.