There can be provided an energy storage device comprising a positive electrode comprising a nitroxyl compound having a nitroxyl cation partial structure in an oxidized state and having a nitroxyl radical partial structure in a reduced state; a negative electrode comprising a carbon material which lithium ions can be reversibly intercalated into and deintercalated from; and an electrolytic solution comprising a lithium salt and an aprotic organic solvent, wherein the negative electrode is a negative electrode comprising the carbon material which lithium ions are previously intercalated into, apart from lithium ions associated with a capacity A of lithium capable of being intercalated and deintercalated in charge and discharge, thereby allowing the energy storage device to simultaneously achieve high energy density, high output characteristics, low environmental load, and high stability in charge and discharge cycles.