1. A method for conducting an electric/magnetic field analysis using a finite different time domain method dividing an analysis space by a polyhedron cell and differentiating a spatial domain, comprising: setting a boundary between materials in the analysis space; specifying a surface on which the boundary exists, out of surfaces constituting a cell used to divide the analysis space; and approximating a boundary existing on the specified surface according to at least one predetermined pattern and conducting an electric/magnetic field analysis calculating both the electric field intensity and magnetic field intensity of the surface.