A MOSFET using a SiC substrate has a problem that a carbon-excess layer is formed on a surface by the application of mechanical stress due to thermal oxidation and the carbon-excess layer degrades mobility of channel carriers. In the invention, (1) a layer containing carbon-carbon bonds is removed; (2) a gate insulating film is formed by a deposition method; and (3) an interface between a crystal surface and the insulating film is subjected to an interface treatment at a low temperature for a short time. Due to this, the carbon-excess layer causing characteristic degradation is effectively eliminated, and at the same time, dangling bonds can be effectively eliminated by subjecting an oxide film and an oxynitride film to an interface treatment.