In a capacitor equivalent circuit model for when a DC voltage is superimposed on a signal, using an equivalent circuit for when the DC voltage is not applied as a baseline, a voltage-controlled current source or voltage source that adjusts the signal in accordance with the superimposed DC voltage is substituted with a portion that represents the capacitance of the equivalent circuit. The voltage-controlled current source or voltage source is then modeled using a current or voltage approximation formula that describes the change in the signal resulting from the changes in the characteristics of the capacitor due to superimposition of the DC voltage, and a device that represents the capacitance of the equivalent circuit behaves similar to a variable device due to superimposition of the DC voltage. This approximation formula reduces computational load on a simulator and prevents issues such as divergence.