A heat-resistant fuel-activating substance is affixed to a combustion device such as a boiler in an adequate manner, that is, the substance is affixed in an adequate position over an adequate area, whereby the effect of activating combustion is rapidly, stably, and inexpensively produced. A heat-resistant fuel-activating substance having a spectral emissivity of 0.85 or higher at electromagnetic wavelengths in the range of 3-20 µm is affixed to a combustion device so that the heat-resistant fuel-activating substance is disposed in a position which is located outside or inside the combustion chamber at the back of the flame-generating portion of the burner and rises to at most 300°C in temperature and that the substance occupies at least 50% of the area of the projected part of the combustion cone.

There is provided a far infrared radiator capable of highly activating methane gas prior to combustion to thereby attain an enhancement of thermal efficiency at combustion. In a passage of methane gas prior to combustion, there is positioned absorbent material (58) produced by mixing at least carbon with tourmaline as an absorption material of absorptivity higher than that of the material of the methane gas passage. Energy-absorbing coating (59) is applied to an fixed on the external surface of the absorbent material, and on a circumferential surface of the energy-absorbing coating there are positioned magnet (54) and far infrared radiator (57) produced by mixing tourmaline with at least iron powder and carbon.