PURPOSE: To obtain a heat-accumulating material resistant to phase-separation, easily made to be soluble in water, having excellent shape-retainability in solidified state and capable of uniformly releasing the heat from the whole surface of the material, by using a heat-accumulating substance composed of a hydrated salt exhibiting a phase-transition at a specific temperature, impregnating the substance in a water-retainable substrate and packaging the product in air-tight state.

CONSTITUTION: The objective heat-accumulating material can be produced by impregnating (A) a heat-accumulating substance composed of a hydrated salt (e.g. sodium acetate trihydrate, sodium thiosulfate pentahydrate, etc.) exhibiting phase-transition at a specific temperature into (B) a water-retainable substrate (e.g. nonwoven fabric, paper, wood chip, etc.) and air-tightly packaging the product.