an acquisition unit that acquires a voltage, a current, and a temperature of a lead-acid battery;

a first deriving unit that derives a first SOC and a second SOC that are SOCs of a start point and an end point of an estimation period;

a second deriving unit that derives a total amount of an overcharge amount that is a difference between an amount of charge and a discharge capacity in the estimation period;

a third deriving unit that derives an actual measurement error based on a difference between the derived first SOC and the derived second SOC and the derived total amount of the overcharge amount;

a first specification unit that specifies an estimation error based on the derived first SOC, the derived second SOC, and the acquired temperature, and a relationship between the first SOC, the second SOC, and the temperature of the lead-acid battery, and the estimation error derived based on the difference between the first SOC and the second SOC and the total amount of the overcharge amount;

a second specification unit that specifies an abnormality degree of the derived actual measurement error based on the derived actual measurement error and the specified estimation error; and

an estimation unit that estimates generation of an internal short-circuit of the lead-acid battery based on the specified abnormality degree.