A method for manufacturing a semiconductor device includes forming a trench in a semiconductor wafer; and forming a first insulating film by thermally oxidizing the semiconductor wafer. The first insulating film covers an inner surface of the trench so that a first space remains in the trench. The first insulating film has a recessed portion at the bottom of the trench. The method further includes forming a semiconductor layer on the first insulating film, the semiconductor layer filling the first space and the recessed portion; forming a second space in the trench by selectively removing the semiconductor layer so that a portion of the semiconductor layer remains in the recessed portion; forming a second insulating film in the recessed portion by thermally oxidizing the portion of the semiconductor layer; and forming a first conductive body in the trench, the first conductive body filling the second space.