A semiconductor device having favorable electrical characteristics is provided. A first oxide is formed over a substrate; a first insulator is formed over the first oxide; an opening reaching the first oxide is formed in the first insulator; a first oxide film is deposited in contact with the first oxide and the first insulator in the opening; a first insulating film is deposited over the first oxide film; microwave treatment is performed from above the first insulating film; heat treatment is performed on one or both of the first insulating film and the first oxide; a first conductive film is deposited over the first insulating film; and part of the first oxide film, part of the first insulating film, and part of the first conductive film are removed until a top surface of the first insulator is exposed, so that a second oxide, a second insulator, and a first conductor are formed. The microwave treatment is performed using a gas containing oxygen under reduced pressure, and the heat treatment is performed under reduced pressure.