In calculating an electromagnetic field intensity by a moment method, real and imaginary parts of a mutual impedance, mutual admittance, and mutual reaction between elements and of the elements themselves are approximated by a sum of terms each with a frequency raised to a power. The mutual impedances, etc. at a plurality of sample frequencies are calculated using known mathematical equations, and simultaneous equations obtained by substituting the sample frequencies and the mutual impedances at the respective sample frequencies into an approximate equation are solved to calculate approximation coefficients for the approximate equation. Using the thus obtained approximate equations, the mutual impedance, etc. at a designated frequency are calculated, and substituted into simultaneous equations describing a boundary condition for each element; by solving the simultaneous equations, the electric current, equivalent electric current, and equivalent magnetic current of each element are calculated, from which the electromagnetic field intensity is calculated.