To solve the problem that hot heat of domestic hot water supply level cannot be stored at a high density.

When a heat accumulation temperature is T, variation in enthalpy in a chemical reaction is ΔH, variation in entropy is ΔS, and variation in free energy is ΔG, a thermal storage material satisfying the relationship of TΔS≥G is used in a condition of ΔH>0 so as to promote a reaction for putting the thermal storage material in a thermal storage reaction part 5 in an energy storing state by having supplemental energy added by an electrode part 14 when putting the thermal storage material in a heat accumulating state by decomposing or separating it.