To provide a semiconductor device with improved reliability. To provide a semiconductor device with stable characteristics. To provide a transistor having a low off-state current. To provide a transistor having a high on-state current. To provide a novel semiconductor device, a novel electronic device, or the like. A method for manufacturing the semiconductor device includes the steps of forming a first semiconductor over a substrate; forming a second semiconductor over and in contact with the first semiconductor; forming a first layer over the second semiconductor; performing oxygen plasma treatment and then removing the first layer to expose at least part of a surface of the second semiconductor; forming a third semiconductor over and in contact with the second semiconductor; forming a first insulator over and in contact with the third semiconductor; and forming a first conductor over the first insulator.