PURPOSE: To maintain good cool feeling for a long time outdoors in a hot season by providing a hydrate agent which contains crystal water and an endothermic agent, mixing both agents so as to cause endothermic reaction.

CONSTITUTION: An endothermic agent and equivalency of hydrate agent are divided. Each divided hydrate agent is thrown in the hydrate agent with time lag. The hydrate agent referred herein include crystal water for mirabilite, sodium carbonate 10 water salts, calcium chloride 6 water salts, which is bonded with a specified bonding force in the configuration of salt molecules and water molecules or coordinated water. Ammonium nitrate, sodium nitrate, potassium nitrate or the like may be cited as an endothermic agent while 25wt of their respective mixtures may be cited in terms of 50wt of the hydrate agent. As a result, the endothermic reaction gradually proceeds and the hydrate agent including crystal water is effective to extend cold insulation time. Especially in hot summer, it can provide good cool feeling outdoors, thereby extending cold insulation time sharply.