To easily and inexpensively store the heat of large capacity while reducing heat radiation loss.

The can body units 27 have sealed pressure-proofing structures of uniform shape, respectively having an upper connection pipe 29 in which a heat medium for heat storage flows in a stratification state on an upper edge of the cylindrical can body 28 long in the height direction in comparison with its diameter, and a lower connection pipe 30 at a lower end. A recessed connection port 31 is formed on an upper edge of the upper connection pipe 29, on the other hand, a projecting connection port 32 is formed on a lower end of the lower connection pipe 30. Three can body units 27 are connected in series to constitute the heat storage tank for heating 1, and two can body units 27 are connected in series to constitute the heat storage tank for hot water supply 16. The upper connection pipe 29 and the lower connection pipe 30, and a connecting part of both pipes 29, 30 are covered by a first insulating material 33, and all of the can bodies 28 are covered by a second insulating material 34 from an outer side of the first insulating material 33 to heat insulation.