A wavelength computation unit computes a wavelength inside a substance based on the values of an analytical model that is set with an analytical model setting unit and values defining a wave source that is set by a wave source setting unit. A coefficients computation unit calculates the coefficients of a second computational formula for computing a polarization current when a grid spacing that is set by a computation conditions setting unit is equal to or larger than " 1/10" of the wavelength inside the substance. When the grid spacing is less than " 1/10" of the wavelength, the coefficients of a first computational formula is computed. A FDTD computation unit computes the first or second computational formula or each component of the electromagnetic field based on the FDTD method from the calculated coefficients.