A computer-implemented method for setting equivalent fissile coefficients for each of predetermined nuclides that in targeting, as a manufacture object, an MOX fuel assembly containing uranium and plutonium as nuclear fuel materials, is used for determining a plutonium enrichment corresponding to a plutonium isotopic composition applied to the nuclear fuel materials by an equivalent fissile method, comprising the steps of: selecting a plurality of plutonium isotopic compositions that cover a range of plutonium isotopic compositions to be applied thereto up to a number larger than the whole number of the equivalent fissile coefficients; determining an enrichment distribution of each MOX fuel rod that becomes equivalent in infinite multiplication factor to a design serving as a predetermined base for each of the selected plutonium isotopic compositions; and obtaining the equivalent fissile coefficients by calculating an equation obtained by applying a least squares method to an equivalent fissile method defined by an equation using a plutonium enrichment ε based on the determined enrichment distribution and data based on a weight ratio that each isotope occupies in uranium or plutonium in each plutonium isotopic composition. ε = ∑ m c m ⋅ w m − E 239 ∑ m c m ⋅ w m − ∑ n c n ⋅ w n Where ε: Enrichment at which k ∞ in EOC reactor core average burn-up becomes a value equal to k ∞ of standard fuel E 239: Enrichment at which k ∞ at EOC reactor core average burn-up becomes a value equal to k ∞ of standard fuel in virtual isotopic composition consisting of only Pu239 and U238, w: Weight ratio that each isotope occupies in uranium or plutonium. Incidentally ∑ m w m = 1, ∑ n w n = 1 c: Equivalent fissile coefficients of each isotope in uranium or plutonium. Incidentally, they are defined as C 28 = 0 and C 49 = 1. m: Subscript indicating each isotope of uranium. Incidentally, they are: 24: U234 25: U235 26: U236 28: U238. n: Subscript indicating each isotope of plutonium (Am241 is included conveniently). Incidentally, they are: 48: Pu238 49: Pu239 40: Pu240 41: Pu241 42: Pu242 51: Am241. selecting a plurality of plutonium isotopic compositions that cover a range of plutonium isotopic compositions to be applied thereto up to a number larger than the whole number of the equivalent fissile coefficients; determining an enrichment distribution of each MOX fuel rod that becomes equivalent in infinite multiplication factor to a design serving as a predetermined base for each of the selected plutonium isotopic compositions; and obtaining the equivalent fissile coefficients by calculating an equation obtained by applying a least squares method to an equivalent fissile method defined by an equation using a plutonium enrichment ε based on the determined enrichment distribution and data based on a weight ratio that each isotope occupies in uranium or plutonium in each plutonium isotopic composition. ε = ∑ m c m ⋅ w m − E 239 ∑ m c m ⋅ w m − ∑ n c n ⋅ w n Where ε: Enrichment at which k ∞ in EOC reactor core average burn-up becomes a value equal to k ∞ of standard fuel E 239: Enrichment at which k ∞ at EOC reactor core average burn-up becomes a value equal to k ∞ of standard fuel in virtual isotopic composition consisting of only Pu239 and U238, w: Weight ratio that each isotope occupies in uranium or plutonium. Incidentally ∑ m w m = 1, ∑ n w n = 1 c: Equivalent fissile coefficients of each isotope in uranium or plutonium. Incidentally, they are defined as C 28 = 0 and C 49 = 1. m: Subscript indicating each isotope of uranium. Incidentally, they are: 24: U234 25: U235 26: U236 28: U238. n: Subscript indicating each isotope of plutonium (Am241 is included conveniently). Incidentally, they are: 48: Pu238 49: Pu239 40: Pu240 41: Pu241 42: Pu242 51: Am241. ε: Enrichment at which k ∞ in EOC reactor core average burn-up becomes a value equal to k ∞ of standard fuel E 239: Enrichment at which k ∞ at EOC reactor core average burn-up becomes a value equal to k ∞ of standard fuel in virtual isotopic composition consisting of only Pu239 and U238, w: Weight ratio that each isotope occupies in uranium or plutonium. Incidentally ∑ m w m = 1, ∑ n w n = 1 c: Equivalent fissile coefficients of each isotope in uranium or plutonium. Incidentally, they are defined as C 28 = 0 and C 49 = 1. m: Subscript indicating each isotope of uranium. Incidentally, they are: 24: U234 25: U235 26: U236 28: U238. 24: U234 25: U235 26: U236 28: U238. n: Subscript indicating each isotope of plutonium (Am241 is included conveniently). Incidentally, they are: 48: Pu238 49: Pu239 40: Pu240 41: Pu241 42: Pu242 51: Am241. 48: Pu238 49: Pu239 40: Pu240 41: Pu241 42: Pu242 51: Am241.