1. A battery degradation determination device comprising: a fitting module configured to fit AC impedance measurement data into an equivalent circuit model including at least one circuit block in which a resistance and a constant phase element are connected in parallel, and to obtain circuit constants in the equivalent circuit model; a P-value saving module configured to save P values being index of the constant phase element obtained by fitting AC impedance measurement data of a reference battery to the equivalent circuit model; and a degradation determination module configured to perform degradation determination for a battery as a determination target based on circuit constants obtained by fitting AC impedance measurement data of the battery as a determination target to the equivalent circuit model with the use of the P values as fixed values, with reference to correlations between degrees of battery degradation and the circuit constants in the equivalent circuit model.