To reproduce electric characteristics of a MOS transistor at high level of accuracy.

A circuit simulation apparatus including a graphic information generation means, a parameter correction amount calculation means and a circuit simulation means performs circuit simulation by the following method. In the method, a middle point in the longitudinal direction of a channel on a boundary between a channel region and an STI region is specified. When a gate width direction is set as a vertical direction, the middle point is set as an origin and a distance between a MOS transistor and a diffusion layer adjacent to the MOS transistor is set as a vertical direction adjacent diffusion layer distance, the vertical direction adjacent diffusion layer distance is specified as a function of a vertical direction distance to be changed according to a position X in the longitudinal direction of the channel. A multiplication expression multiplying the function of the vertical direction adjacent diffusion layer distance and a weighting function is generated and a parameter correction amount is calculated on the basis of the multiplication expression.

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