[Solving Means] A resonance calculation program is configured to calculate an effective cross section including Step S4 of setting a calculation point in the gray range in the resonance region; Step S5 of calculating a neutron flux set at the calculation point and corresponding to a macroscopic cross section based on Method of Characteristics; Step S6 of fitting a fitting equation to the calculated macroscopic cross section and the calculated neutron flux, and calculating first and second rational coefficients; Step S7 of calculating a background cross section based on the first rational coefficient; Step S8 of interpolating the effective cross section, with the background cross section used as an argument; Step S9 of calculating the neutron flux based on the background cross section; and Step S10 of calculating the effective cross section based on the effective cross section, the neutron flux, and the second rational coefficient obtained at the coefficient calculation step.