The present invention relates to a high-capacity electrochemical energy storage device that advances a conversion reaction as an oxidation-reduction reaction and has suppressed electrode potential deviation (hysteresis) between oxidation and reduction. The device comprises a first electrode including a first active substance, a second electrode including a second active substance, and a non-aqueous electrolyte interposed between the first electrode and the second electrode. At least one of either the first active substance or the second active substance is a metal salt having a polyatomic anion and a metal ion. The metal salt is capable of an oxidation-reduction reaction having reversible exchange of polyatomic anions.