The present invention provides a method to calculate refinement parameters from an observed diffraction pattern for powder samples accurately. A method to calculate a best solution of the crystal structural parameters from a diffraction pattern, comprising: a third calculating step of the converged values 600 to calculate at least three converged values; a third judging step of the best converged values 700 to calculate at least three criteria from the peak-shift parameters in the converged values and to judge whether the converged values are a true solution of not by using the criteria; and a first calculating step of a global solution 800 to calculate a global solution of which is the true value by using the criteria.