A silicon carbide (SiC) semiconductor device with a metal-oxide-semiconductor structure, the device comprising: a SiC substrate; a gate electrode made from polycrystalline silicon (Si); a gate oxide film formed by thermally-oxidizing the SiC substrate, the gate oxide film is disposed between the SiC substrate and the gate electrode, and connected to the SiC substrate and the gate electrode, the thickness of the gate oxide film is 20 nm or less; an ohmic contact electrically connected to the SiC substrate; and a polycrystalline silicon thermally-oxidized film formed by oxidizing a surface of the gate electrode, wherein the silicon carbide (SiC) semiconductor device is obtained by: increasing a temperature to at least a predetermined temperature while maintaining the SiC substrate in an inert gas atmosphere; when the temperature rises to at least the predetermined temperature, forming the gate oxide film by thermally-oxidizing the SiC substrate by introducing an oxidation gas; when a thickness of the gate oxide film becomes a predetermined thickness, stopping the introduction of the oxidation gas and reestablishing the inert gas atmosphere; when the inert gas atmosphere is reestablished, decreasing the temperature and stopping the thermal oxidation of the SiC substrate, and forming the gate electrode on a surface of the gate oxide film by depositing polycrystalline silicon on the gate oxide film, wherein thermally-oxidizing the SiC substrate is performed at a temperature higher than any other temperature in subsequent processing steps, further comprising forming a polycrystalline silicon thermally-oxidized film by oxidizing a part of a surface of the gate electrode; and: forming an interlayer insulating film on the gate oxide film and the polycrystalline silicon thermally-oxidized film; or forming an interlayer insulating film on the polycrystalline silicon thermally-oxidized film and an exposed part of at least one of the gate oxide film and a field insulating film. a SiC substrate; a gate electrode made from polycrystalline silicon (Si); a gate oxide film formed by thermally-oxidizing the SiC substrate, the gate oxide film is disposed between the SiC substrate and the gate electrode, and connected to the SiC substrate and the gate electrode, the thickness of the gate oxide film is 20 nm or less; an ohmic contact electrically connected to the SiC substrate; and a polycrystalline silicon thermally-oxidized film formed by oxidizing a surface of the gate electrode, wherein the silicon carbide (SiC) semiconductor device is obtained by: increasing a temperature to at least a predetermined temperature while maintaining the SiC substrate in an inert gas atmosphere; when the temperature rises to at least the predetermined temperature, forming the gate oxide film by thermally-oxidizing the SiC substrate by introducing an oxidation gas; when a thickness of the gate oxide film becomes a predetermined thickness, stopping the introduction of the oxidation gas and reestablishing the inert gas atmosphere; when the inert gas atmosphere is reestablished, decreasing the temperature and stopping the thermal oxidation of the SiC substrate, and forming the gate electrode on a surface of the gate oxide film by depositing polycrystalline silicon on the gate oxide film, increasing a temperature to at least a predetermined temperature while maintaining the SiC substrate in an inert gas atmosphere; when the temperature rises to at least the predetermined temperature, forming the gate oxide film by thermally-oxidizing the SiC substrate by introducing an oxidation gas; when a thickness of the gate oxide film becomes a predetermined thickness, stopping the introduction of the oxidation gas and reestablishing the inert gas atmosphere; when the inert gas atmosphere is reestablished, decreasing the temperature and stopping the thermal oxidation of the SiC substrate, and forming the gate electrode on a surface of the gate oxide film by depositing polycrystalline silicon on the gate oxide film, wherein thermally-oxidizing the SiC substrate is performed at a temperature higher than any other temperature in subsequent processing steps, further comprising forming a polycrystalline silicon thermally-oxidized film by oxidizing a part of a surface of the gate electrode; and: forming an interlayer insulating film on the gate oxide film and the polycrystalline silicon thermally-oxidized film; or forming an interlayer insulating film on the polycrystalline silicon thermally-oxidized film and an exposed part of at least one of the gate oxide film and a field insulating film. forming an interlayer insulating film on the gate oxide film and the polycrystalline silicon thermally-oxidized film; or forming an interlayer insulating film on the polycrystalline silicon thermally-oxidized film and an exposed part of at least one of the gate oxide film and a field insulating film.