An estimation device for estimating an internal state of an energy storage device having a single electrode containing an active material in which repeated charge-discharge changes a first characteristic that is an energy storage amount-potential charge characteristic, and a second characteristic that is an energy storage amount-potential discharge characteristic, the estimation device comprising: a controller that is configured to acquire at least one actual measured energy storage amount characteristic, which is a third characteristic that is an energy storage amount-voltage charge characteristic or a fourth characteristic that is an energy storage amount-voltage discharge characteristic, in a predetermined voltage range of the energy storage device; and a storage unit that is configured to store a plurality of predetermined energy storage amount characteristics that are at least any of first characteristics, second characteristics, third characteristics, fourth characteristics, and pieces of V-dQ/dV in correspondence with a change in a feature value, which is changed by repeated charge-discharge, or to store the plurality of predetermined energy storage amount characteristics as a function of the feature value, wherein the feature value is correlated with the plurality of predetermined energy storage amount characteristics; wherein the controller is further configured to obtain the feature value from the at least one actual measured energy storage amount characteristic, refer to the plurality of predetermined energy storage amount characteristics or the function in accordance with the feature value to determine an estimated energy storage amount characteristic on the basis of the obtained feature value; and estimate a parameter indicating the internal state of the energy storage device by performing optimization calculation on the basis of the at least one actual measured energy storage amount characteristics and the estimated energy storage amount characteristics. a controller that is configured to acquire at least one actual measured energy storage amount characteristic, which is a third characteristic that is an energy storage amount-voltage charge characteristic or a fourth characteristic that is an energy storage amount-voltage discharge characteristic, in a predetermined voltage range of the energy storage device; and a storage unit that is configured to store a plurality of predetermined energy storage amount characteristics that are at least any of first characteristics, second characteristics, third characteristics, fourth characteristics, and pieces of V-dQ/dV in correspondence with a change in a feature value, which is changed by repeated charge-discharge, or to store the plurality of predetermined energy storage amount characteristics as a function of the feature value, wherein the feature value is correlated with the plurality of predetermined energy storage amount characteristics; wherein the controller is further configured to obtain the feature value from the at least one actual measured energy storage amount characteristic, refer to the plurality of predetermined energy storage amount characteristics or the function in accordance with the feature value to determine an estimated energy storage amount characteristic on the basis of the obtained feature value; and estimate a parameter indicating the internal state of the energy storage device by performing optimization calculation on the basis of the at least one actual measured energy storage amount characteristics and the estimated energy storage amount characteristics.