The present invention enables derivation of a neutron soft error rate only from data at the time of low neutron energy irradiation. An outline value of an SEU cross section function corresponding to a given neutron energy value is output. From the outline value of the SEU cross section function and low energy neutron spectrum data, an error count basic value of errors that occur per unit time is calculated. From an error count at the time of low energy neutron irradiation and the low energy neutron irradiation time, an error count actual measurement value per unit time is calculated. From the error count basic value per unit time and the error count actual measurement value per unit time, a proportionality coefficient of the SEU cross section function is calculated. A natural neutron spectrum is held, and a neutron flux corresponding to the neutron energy value transmitted from the error rate calculation unit is output. An integration operation is performed for the SEU cross section function, which is determi
