An accumulator and state of charge evaluation method for preparing, during charging and discharging, models for each of deterioration degrees, each representing a relationship between terminal voltage, current and state of charge, for selecting respective models for the times during charging and discharging in accordance with a total charging/discharging amount (Ah), a deterioration degree or an elapsed time of an operation of the accumulator, for further estimating state of charge from current accumulation, and for determining a final state of charge from estimated states of charge including a present state of charge estimated from the models for the times during charging and discharging of the accumulator, while using a time or a total charging/discharging amount (Ah) from a time of full-charging by equal charging of the accumulator up to present time to determine the final state of charge from the plurality of states of charge.