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