To solve the defect that the amount of waste heat is decreased and the level varies since the utilization of the amount of heat of the exhaust gas itself is limited in the boiler utilizing the waste heat of the incineration furnace or the melting furnace.

In the boiler utilizing the waste heat of incineration and melting, when the amount of exhaust heat in addition to the amount of generation of the exhaust gas decreases, a fixed amount of heat is constantly given to the boiler for improving the boiler efficiency by additionally burning kerosene, heavy oil, propane, natural gas, or the like, thus restraining the output variation to make its output constant.