In a mutual immittance calculation apparatus, an input section inputs data of a model of an electric circuit apparatus, being a target for analysis of the electromagnetic-field strength and being divided into a plurality of patches. A mutual immittance calculation section calculates respective mutual immittance for combinations of patches corresponding to the main portion and to the additional portion. The mutual immittance calculation section uses a stored calculation result corresponding to the main portion when the model in which only the additional portion has been changed is calculated for a second time onward, and recalculates the mutual immittance corresponding to the changed additional portion.