In general, with fuels used in internal combustion engines, external combustion engines, and other combustion apparatus, exhaust gases are generated by combustion to release various types of noxious chemical substances into the atmosphere, thus causing an environmental pollution. These noxious chemical substances include, for example, CO, HC, NOx, and SOx. Heretofore, to suppress generation of such noxious chemical substances, fuel combustion efficiency has been improved by improving the quality of fuels used or by improving various combustion devices. In addition, the exhaust gases released have been treated by a variety of exhaust gas removing devices using catalysts to prevent pollution. However, such prior art methods have been insufficient to reduce pollutive substances, and the improvement of fuel quality has been difficult in view of the costs. To overcome the defects, recently, the industry intends to use fuel activation devices to enhance combustion and reduce harmful gases. In the U.S. Pat. Nos. 4,605,498, 4,711,271, 5,055,189, 5,044,346, 5,080,080, 5,460,144, 5,632,254 and 5,873,353, magnetic field effect or far infrared ray is used to activate fuel to decompose the fuel molecule and therefore enhance combustion and reduce the harmful gases.