An estimation unit that estimates an estimated remaining service life of the lead storage battery implements: a storage step of storing an integral discharge threshold A that is a reference for determination of a service life of the lead storage battery under a standard condition with predetermined values of a surface temperature, a charge current, and a discharge current of the lead storage battery; a calculation step of calculating an integral discharged capacity B that represents an accumulated value of a discharged capacity of the lead storage battery under the actual conditions such as giving deterioration that same order as that occurring when the lead storage battery is repeatedly discharged under the standard conditions; a subtraction step of obtaining a differential electric quantity A−B by subtracting the integral discharged capacity B from the integral discharge threshold A; and an estimation step of estimating the estimated remaining service life of the lead storage battery by using the differential electric quantity.