PURPOSE: To enable a positive and stable control over a starting of crystallization as well as a keeping of over-cooled state to be attained and further to enable a repetitive use for a long period of time to be attained by a method wherein a trigger is mounted at a surface of a plate in such a way as its concave surface may face against the plate, the trigger is filled within a water-tight shielding and aeration-tight packaging material and then vacuum sealed in it.

CONSTITUTION: Immersed base material 2 made of sheet-like non-woven fabric of organic material having moisture containing characteristic is filled with thermal accumulating substance having aqueous salt and a trigger 3 in which a metallic piece is bent on the surface of the base member and formed into a semi-spherical shape is sealingly packaged under vacuum condition in a bag 1 of sealed container laminated with soft synthetic resin material in order to improve a gas barrier characteristic. In case of this thermal accumulation material 4, as the thermal accumulating substance, sodium acetate 3 aqueous salt is employed, thermal accumulators are set at both sides, thermal keeping materials 5 and 6 made of urethane foam are held and stored in an outer cloth bag 7 to form a cushion 8. As a result, after the thermal accumulation members are over-cooled after heating, then the trigger 3 is applied with a light pressure of a finger over the bag 1 after 24 hours, then the trigger function may act to discharge a latent heat of 58°C and the thermal radiation is continued for a long period of time.