To form a MOSFET over a silicon carbide substrate, when a heat treatment accompanied by nitration is carried out to reduce the interface state density in the vicinity of the boundary between a gate insulating film and a silicon carbide substrate, CV hysteresis occurs due to the relationship between the capacitance and gate voltage of the MOSFET, thereby reducing the reliability of a semiconductor device.To solve the above problem, a heat treatment accompanied by nitration is carried out on the insulating film formed over the silicon carbide substrate (step S7). Then, the insulating film is heated in an inert gas atmosphere (step S9). Thereafter, a field effect transistor having a gate insulating film which is composed of the insulating film is formed over the silicon carbide substrate.