During charging of a secondary battery, a rate of voltage change dV/dt is obtained when a voltage V of the secondary battery reaches a predetermined voltage Vpre, a charge current I for charging the secondary battery is detected, and a rate of change dQ/dt in a quantity of charged electricity Q of the secondary battery is calculated on the basis of the result of the detection. Here, the predetermined voltage Vpre is a voltage higher than a discharge cutoff voltage and lower than a charge cutoff voltage. Thereafter, a ratio X: dQ/dV, of the rate of change dQ/dt in the quantity of charged electricity to the rate of voltage change dV/dt is calculated. The calculated ratio X is compared with a reference value Xref, and degradation of the secondary battery is determined on the basis of the result of the comparison.