PURPOSE: To obtain the heat accumulating material, whose overcooling is prevented from occurring by a method wherein disodium hydrogen phosphate is employed as main component and at least either one of muscovite type mica and asbestos is contained as nucleus agent.

CONSTITUTION: Disodium hydrogen phosphate is used as main component of the heat accumulating material and as well as either single or both of muscovite type mica such as muscovite, sericite, illite or the like and asbestos are used as nucleus agent. Especially because that asbestos has a property to uniformly disperse itself in molten disodium hydrogen phosphate, the state, in which nuclei are uniformly dispersed, is easily realized in system. Consequently, the heat accumulating capacity of the heat accumulating material is enlarge and, moreover, due to the addition of the nucleus agent, the overcooling of the heat accumulating material is prevented and consequently the heat absorption at constant temperature is realized without difficulty, resulting in enabling to stabilize the performance of the heat accumulating material.