The drift region for increasing the breakdown voltage in an LDMOSFET is regarded as a resistive element. The potential distribution of the overall device is calculated by obtaining a potential distribution considering the resistance by iterative calculation. A capacitance generated in the drift region is analytically calculated assuming a linear potential distribution. A capacitance generated in the overlap region between the gate electrode and the drift region is calculated by considering the potential from the depletion region to the accumulation region.