PURPOSE: To ferment an organic matter and to make use of heat produced at the same time for heating water, by a method wherein a water pipe or a water tank is buried in an organic matter stack, which is covered and enclosed completely by providing a vent hole partly.

CONSTITUTION: An enclosure which is an external wall made of a heat insulating material is installed to stack an organic matter becoming a heat source and to make the most of the same simultaneously with prevention of radiation of heat, within which an article formed in a cylindrical shape by winding up a pipe spirally is fixed by connecting the lower and upper ends of the same with a water supply and hot water supply openings respectively. Although it is necessary to add water and a temperature for fermentation and water content to an extent of green grass is good enough for the water, it is unnecessary to be paticular about a precise figure of the temperature along with the temperature. The upper part of the enclosure in which the organic matter is packed in this manner, that is, the ceiling is made sunny by covering the same with a vinyl sheet and heat accumulation is made by absorbing solar heat. Several days later the water in the pipe becomes hot water by hot air obtained through fermentation, and the hot water is kept at a temperature of about 20W50°C even in winter until a stacked matter turns into leaf mold.