The authors have developed the inclusive technique for the improvement of thermal efficiency by "spectrum heat absorption method" using the electro-magnetic wave at specific band of far infrared rays for realization of high energy saving on fossil fuel combustion. The spectrum heat absorption method makes it possible to increase temperature rise ratio between fluid gas and combustion gas by absorbing the radiation spectrum of carbon dioxide (CO_2) produced from high temperature combustion gas through the Aprm radiant materials applied in the heat exchange region. From several kinds of verification tests we may predict that a specific wave number of far infrared rays has an ability to increase the amount of heat transfer to working fluids from combustion exhaust gas. The principle is very useful and effective to achieve the high energy saving on fossil fuel combustion.