A prestored database indicating a correspondence of δ (depth)/ΔHcJ (coercivity increment) and a prestored database indicating information of Dy diffusion condition (diffusion coefficient, diffusion flux and processing time regarding Dy diffusion) are used to compute distribution of Dy introduction amount in a magnet from shape information of the magnet and information of a Dy introduction face and compute distribution of ΔHcJ in the magnet from the distribution of Dy introduction amount. Regarding a magnet having a coercivity that is distributed non-uniformly, a J/H curve is computed using the computed distribution of ΔHcJ, and a demagnetizing factor at a predetermined temperature is computed using a temperature coefficient.