PURPOSE: To obtain a process/device simulation device which calculates a highly precise solution in a short time.

CONSTITUTION: A discrete grating generating means 1 sets grating points by generating discrete gratings. A grating point limiting means 4 limits grating points where solutions in an initial stage should be found among all the grating points. A reciprocal calculating means 2a gives an optional initial value and performs the reciprocal calculation as to the grating points that the grating point limiting means 4 limits until the solutions are converged to find the solutions in the initial stage respectively. An interpolating means 5 find initial- stage solutions of grating points that the grating point limiting means 4 does not limit by using interpolation. Further, a repeated calculating means 2a finds final solutions by repeated calculation as to all the grating points by using the solutions, found in the initial stage by the repeated calculation and the solutions obtained by the interpolation. Consequently, the frequency of calculation in the initial stage can be made less than that when repeated calculation is performed from the beginning as to all the grating points by giving an optional initial value.