A method of controlling the input and output of heat to and from a heat-storing material. The method includes the step of externally supplying thermal energy to the heat-storing material capable of being supercooled and filled into a plurality of small containers together with a phase-segregation preventive agent by use of a structure for supplying heat, whereby the heat-storing material is melted. The method also includes the steps of maintaining the heat-storing material in the supercooled state after the emission of sensible heat, and releasing the supercooled state of the heat-storing material by use of a structure for releasing the supercooled state of the heat-storing material, when the release of the stored heat energy is required, whereby the heat at the melting point is generated.