To remove dissolved oxygen in heat storage water in a heat storage tank by an inexpensive and simple operation so as to prevent piping for cooling and heating of a building from rusting by forming a heat storage tank storing heat storage water as a closed chamber and always filling a space above a water face within the heat storage tank with nitrogen gas.

Heat storage water is stored within a stratification thermal storage (vertical single tank type) tank 1 and openings 2, 3 for heating/cooling and return pipes are formed in an upper end and a lower end of a side face of the tank 1 and heat storage water is sucked from one opening of the pipe at night at a low speed by a suction pump 8 to supply the heat storage water to a heat source machine and is returned through the other opening of the pipe at a low speed after being cooled in summer and being heated in winter. A top opening of the thermal storage tank 1 is covered with a sealable upper lid with a packing or the like to form an easy closed chamber and nitrogen gas introduced by a nitrogen feeder 6 is stored in a space above the heat storage water and dissolved oxygen in the heat storage water is dissipated into the nitrogen gas according to Henry's law.