A magnetic force characteristic computing method implemented on a computer for obtaining a magnetic force characteristic in a magnet prepared by introducing a heavy rare-earth element from a surface of a magnet and diffusing the heavy rare-earth element into an outer periphery of the main phase or in a magnet prepared by diffusing a heavy rare-earth element into inside of a magnet, the method being implemented on a computer and comprises: preliminarily storing depth δ/ coercivity increment ΔHcJ characteristic information, δ/ΔHcJ, on distribution of an increment of a coercivity due to diffusion of a heavy rare-earth element against depth δ from a surface of a magnet and introduction amount Dy/coercivity increment ΔHcJ characteristic information, Dy/ΔHcJ, on a characteristic of an increment of a coercivity due to diffusion against introduction amount of a heavy rare-earth element; a first step of obtaining δ/Dy information on Dy distribution of a heavy rare-earth element against δ from the stored δ/ΔHcJ and the stored Dy information; a second step of accepting shape information on a dimension and a shape of the magnet;a third step of accepting introduction face information on one or a plurality of faces where a heavy rare-earth element has been introduced, in accordance with the accepted shape information;a fourth step of computing Dy distribution in the magnet of a heavy rare-earth element introduced from the one or a plurality of faces for each face, based on the accepted shape information, the accepted introduction face information and the obtained δ/Dy information;a fifth step of obtaining Dy distribution in an entire magnet from Dy distribution obtained for each face in the fourth step; and a sixth step of obtaining distribution of a ΔHcJ due to diffusion in the magnet, based on the Dy distribution in the entire magnet obtained in the fifth step and the stored Dy/ ΔHcJ information, characterized in that Dy distribution against depth δ exceeding a distance from a face of the magnet where a heavy rare-earth element is introduced to a rear face in the Dy distribution of a heavy rare-earth element against depth δ is folded back at the rear face functioning as a symmetry plane and added repeatedly until no difference remains in introduction amount distribution in the fourth step. preliminarily storing depth δ/ coercivity increment ΔHcJ characteristic information, δ/ΔHcJ, on distribution of an increment of a coercivity due to diffusion of a heavy rare-earth element against depth δ from a surface of a magnet and introduction amount Dy/coercivity increment ΔHcJ characteristic information, Dy/ΔHcJ, on a characteristic of an increment of a coercivity due to diffusion against introduction amount of a heavy rare-earth element; a first step of obtaining δ/Dy information on Dy distribution of a heavy rare-earth element against δ from the stored δ/ΔHcJ and the stored Dy information; a second step of accepting shape information on a dimension and a shape of the magnet;a third step of accepting introduction face information on one or a plurality of faces where a heavy rare-earth element has been introduced, in accordance with the accepted shape information;a fourth step of computing Dy distribution in the magnet of a heavy rare-earth element introduced from the one or a plurality of faces for each face, based on the accepted shape information, the accepted introduction face information and the obtained δ/Dy information;a fifth step of obtaining Dy distribution in an entire magnet from Dy distribution obtained for each face in the fourth step; and a sixth step of obtaining distribution of a ΔHcJ due to diffusion in the magnet, based on the Dy distribution in the entire magnet obtained in the fifth step and the stored Dy/ ΔHcJ information, characterized in that Dy distribution against depth δ exceeding a distance from a face of the magnet where a heavy rare-earth element is introduced to a rear face in the Dy distribution of a heavy rare-earth element against depth δ is folded back at the rear face functioning as a symmetry plane and added repeatedly until no difference remains in introduction amount distribution in the fourth step.