An SOC estimation device 50 of an energy storage device includes a storage unit 73 and a data processing unit 71. The energy storage device 100 has a characteristic including a first deterioration mode in which a capacity drop with respect to time indicates a first transition, and a second deterioration mode in which a capacity drop indicates a second transition. The storage unit 73 holds first correlation data M1 indicating a correlation between SOC and OCV of the energy storage device in the first deterioration mode, and second correlation data M2 indicating a correlation between SOC and OCV of the energy storage device in the second deterioration mode. The data processing unit 71 executes a mode determination process of determining a deterioration mode of the energy storage device, and an estimation process of selecting correlation data corresponding to the deterioration mode from the storage unit, to estimate SOC of the energy storage device.