To save limited resources and reduce users' expenses by using heat produced as a byproduct of the decaying decomposition of plants by the action of yeast.

Since the temperature of heat produced as a byproduct of the decaying decomposition of plants (including waste and wood chips, withered grass and garbage) by yeast and microorganisms reaches a high temperature about 60 degrees, in a water heater, a heat-resistant and heat-insulated yeast tank has a heat exchanger 3, a lid and vents 5 to use the heat as the heat source. Plants and easy-to-handle yeast are mixed in the yeast tank, water and sodium chloride are input to maintain an optimum humidity for promoting the activity of the yeast, and heat produced by decaying decomposition by the action of the yeast is input into the heat exchanger, which in turn generates hot water.