PURPOSE: To control the cooling effect and to enable a low-temp. state to be maintained over a prolonged period of time by coating a substance which brings about an endothermic reaction upon being dissolved in water with a dissolution- controlling film.

CONSTITUTION: A grain having a size of pref. 2W5mm of a substance which brings about an endothermic reaction upon being dissolved in water, such as urea or ammonium nitrate, is coated with a dissolution-controlling film composed of a polyolefin (most pref., low-density polyethylene) and, according to necessity, a water-soluble polymer, such as a copolymer of ethylene with vinyl acetate or polyethylene oxide, and a water-insoluble fine mineral particle (most pref., talc in an amt. of 50W95wt.%), thereby obtaining a persistent coolant. The above coating makes it possible to freely change the water dissolution rate of the endothermic substance by changing the compsn. proportions and the thickness of the film. Thus, it is possible to freely change the duration of the cooling effect.