To obtain a heat or cold storage material that does not use artificially synthesized materials, is easy to produce and profitably recyclable.

Seawater is pumped up by a seawater pump 10. The pumped-up seawater undergoes high temperature sterilization with a sunlight collection panel 26 and is concentrated to a salt concentration necessary to freeze in a specific temperature range below the ice point. The concentrated seawater is packed in a sealed pack and cooled to a specific temperature below the ice point by means of an outside temperature and the heat radiation action of the earth.