A storage amount estimation device that is configured to estimate an electricity storage amount of an energy storage device, wherein at least one of a positive electrode plate and a negative electrode plate of the energy storage device includes an active material, wherein the active material generates at least two electrochemical reactions (A, B) depending on a change of state of charge, and wherein a hysteresis of a voltage value depending on a charge and discharge history in an electricity storage amount-voltage value characteristic during generation of one of the electrochemical reactions is smaller than a hysteresis of the voltage value during generation of the other electrochemical reaction in the active material, the storage amount estimation device comprising: a holding unit that is configured to hold a plurality of electricity storage amount-voltage value characteristics from a lower limit voltage value of a voltage value region, where the reaction amount of the one electrochemical reaction is larger than the reaction amount of the other electrochemical reaction, to a plurality of reached voltage values; and a processing unit that is configured to: • acquire a voltage value of the energy storage device; • set a reached voltage value, when it is determined that the acquired voltage value exceeds the lower limit voltage value, wherein the reached voltage value is a maximum voltage value that has been acquired during charging of the energy storage device; • select one electricity storage amount-voltage value characteristic based on the set reached voltage value; and • estimate the electricity storage amount based on the selected one electricity storage amount-voltage value characteristic and the voltage value acquired by the voltage acquisition unit. a holding unit that is configured to hold a plurality of electricity storage amount-voltage value characteristics from a lower limit voltage value of a voltage value region, where the reaction amount of the one electrochemical reaction is larger than the reaction amount of the other electrochemical reaction, to a plurality of reached voltage values; and a processing unit that is configured to: • acquire a voltage value of the energy storage device; • set a reached voltage value, when it is determined that the acquired voltage value exceeds the lower limit voltage value, wherein the reached voltage value is a maximum voltage value that has been acquired during charging of the energy storage device; • select one electricity storage amount-voltage value characteristic based on the set reached voltage value; and • estimate the electricity storage amount based on the selected one electricity storage amount-voltage value characteristic and the voltage value acquired by the voltage acquisition unit. • acquire a voltage value of the energy storage device; • set a reached voltage value, when it is determined that the acquired voltage value exceeds the lower limit voltage value, wherein the reached voltage value is a maximum voltage value that has been acquired during charging of the energy storage device; • select one electricity storage amount-voltage value characteristic based on the set reached voltage value; and • estimate the electricity storage amount based on the selected one electricity storage amount-voltage value characteristic and the voltage value acquired by the voltage acquisition unit.