Neutron soft error rate derivation is calculated from data at the low energy neutron radiation. An outline value of an SEU cross-section function corresponding to a given neutron energy value is outputted. This outline value and the low energy neutron spectrum data are used to calculate an error count basic value of errors to occur over time. An error count actual measurement value over time is calculated from an error count during radiation of low energy neutrons and low energy neutron radiation time. The error count basic value and the error count actual measurement are used to calculate a proportionality coefficient of the SEU cross-section function. While holding a natural neutron spectrum, an error rate calculator outputs a neutron flux corresponding to a neutron energy value. The neutron soft error rate is calculated by an integration operation of multiplying the SEU cross-section function with the natural neuron spectrum.