A semiconductor substrate (1,2) made of a semiconductor material is prepared, and a hetero semiconductor region (3) is formed on the semiconductor substrate (1,2) to form a heterojunction in an interface between the hetero semiconductor region (3) and the semiconductor substrate (1,2). The hetero semiconductor region (3) is made of a semiconductor material having a bandgap different from that of the semiconductor material, and a part of the hetero semiconductor region (3) includes a film thickness control portion whose film thickness is thinner than that of the other part thereof. By oxidizing the hetero semiconductor region (3) with a thickness equal to the film thickness of the film thickness control portion, a gate insulating film (4) adjacent to the heterojunction is formed. A gate electrode (5) is formed on the gate insulating film (4). This makes it possible to manufacture a semiconductor device including the gate insulating film (4) with a lower ON resistance, and with a higher insulating characteristic and reliability.