PURPOSE: To prevent the occurrence of errors and mistakes on a numerical calculation and to improve efficiency in calculating, in an optimizing calculation used for an optical system(lens) design.

CONSTITUTION: This method is a descending method among an optimizing method, or one of the descending method, a DLS method, and this method is prepared for avoiding division which is excessive on the numerical calculation by giving each element of Jacobi matrix A not by a ratio of evalution function variation ΔF to variable variation ΔX; ΔT/ΔX. but only by ΔF. By having such a way, an adverse effect due to the division by ΔX is prevented, so that the efficiency in calculation is improved.