PURPOSE: To prevent the deterioration of accuracy due to digit dropping in the impedance calculation and to calculate with high accuracy at a high speed in terms of an electromagnetic field intensity calculation device which calculates an electromagnetic field intensity that an electric circuitry device emits by utilizing a moment method.

CONSTITUTION: The title device comprises a first calculation operation section 12 that calculates a mutual impedance by using an approximation equation wherein a part of an exponential function is equivalent to a constant in the integration of a monopole and a second calculation operation section 13 that calculates the mutual impedance by using a normal equation. An adopting condition on the basis of an electric length of an element subjected to the calculating of the mutual impedance and an electric length of a distance between the elements is judged by means of a calculation method selection section 11 so that determination is made as to which of the first calculation operation section 12 or second calculation section 13 is selected.