Method includes specifying a first relationship between power supply voltage in a semiconductor device and the maximum operable frequency of internal circuit upon fluctuation of the power supply voltage being detected by changing the power supply voltage in the semiconductor device and the operating frequency of the internal circuit, specifying second relationship between the maximum operable frequency of the internal circuit and the mounting manner of the decoupling capacitor upon the fluctuation of the power supply voltage being detected by changing the mounting manner of the decoupling capacitor and the operating frequency of the internal circuit, and calculating the fluctuation amount of the power supply voltage corresponding to the decoupling capacitor mounting manner based on the first relationship and the second relationship.