An end-of-discharge voltage correction section subtracts, from an end-of-discharge open voltage, the product I×RO of a discharge current I of a block and an ohmic resistance RO of the block and the product ITD×RP of a discharge current ITD obtained by performing a time delay process on the discharge current and a polarization resistance RP of the block. Then, a resulting value is set as an end-of-discharge voltage VL. The ohmic resistance RO is increased as an equivalent cycle count CY increases, and is increased as a healthy parallel number NPH decreases. An increment ΔCY of the equivalent cycle count CY in each charge/discharge cycle becomes greater as a discharge depth DD increases. The healthy parallel number NPH is derived by multiplying a healthy parallel number NPHR of a reference block by the ratio of a capacity CPR of the reference block to a capacity CPS of a target block.