1. A radioactivity evaluation method of evaluating radioactivity on each object sections irradiated with neutrons in a neutron generating facility by means of a radioactivity evaluation system including a calculating section, a memory section, an input section and an output section, the method comprising: a first input step in which the input section reads in a neutron energy spectrum at each position of the object sections, an irradiation period and a cooling period and composition of each material of the object sections; a selection step in which the calculating section selects a representative neutron energy spectrum and a representative neutron fluence rate from the neutron energy spectra at each position of the object sections read in the first input step; a second input step in which the calculating section reads in the representative neutron energy spectra and the representative neutron fluence rates selected in the above selection step; a radioactivity calculating step in which the calculating section calculates quantities of the radioactivity that correspond to the representative neutron energy spectra and the representative neutron fluence rates based on the neutron energy spectra at each position of the object sections, the irradiation period and the cooling period, and the composition of the material of each object sections read in the first input step and the representative neutron energy spectra and the representative neutron fluence rate read in the second input step; a data storing step in which the memory section stores the quantities of the radioactivity calculated in the radioactivity calculating step as radioactivity data; a nuclide-by-nuclide radioactivation reaction rate calculating step in which the calculating section calculates a radioactivation reaction rate of each nuclide based on the neutron energy spectra at each position of the object sections read in the first input step and the radioactivity data stored in the data storing step; an object-by-object adding-up step in which the calculating section adds up the radioactivation reaction rates of constituting nuclides calculated for each of the object sections in the nuclide-by-nuclide radioactivation reaction rate calculating step; a nuclide determining step in which the calculating section determines if the radioactivation reaction rates of the constituting nuclides are added up in the object-by-object adding-up step or not; and an object position determining step in which the calculating section determines if the adding-up operation is performed for the positions of the object sections or not.