A novel diffraction condition simulation device for calculating a reciprocal lattice to a crystal sample and displaying, on a computer screen, a Bragg reflection condition of X rays caused by the crystal sample, characterized in that a section where a plurality of reciprocal lattice points in a limiting sphere which rotates with rotation of a crystal intersect a diffraction plane is displayed with a section of the limiting sphere on the computer screen, is provided, thereby realizing quick and easy calculation and display of a desired Bragg reflection satisfying various diffraction conditions necessary for structure analysis and characterization of structure of a crystal, without any limitation as to diffraction conditions, diffraction information and the like.