To improve convergence of circuit simulation by appropriately setting the transistor initial condition to improve convergence.

Electrostatic capacitor between each terminal and a body of each partial depletion type MOS transistor to be operated for floating in a body area thereof and contained in a circuit to be simulated is obtained (S3). Body electrical potential in the condition that the transistor is turned on for normal operation is obtained on the basis of a formula of a current balance per each transistor (S4), and initial body change quantity is obtained per each transistor on the basis of a relation formula between the electrostatic capacity, an electrical potential of each terminal and the body potential when the transistor is turned on (S5). The initial current flowing in each terminal of each transistor is set at 0 (S6). The initial potential of each terminal is obtained from a net list, and the initial body potential is decided on the basis of the initial potential and the initial body charge quantity (S7).