A method for cleaning exhaust gas for internal combustion engines or combustion equipment is provided, which comprises the steps of: irradiating air, which is introduced through the air inlet of the engine or equipment, with .alpha.-rays by alpha-decay thereof, to thereby transform diatomic oxygen in the air into a powerful oxidizing active oxygen while transforming, by fission, a part of nitrogen in the air into a monatomic oxygen and monatomic hydrogen, and supplying the combustion equipment or internal combustion engine with the air containing the active oxygen, monatomic oxygen and hydrogen to reduce toxic components from the exhaust gas. An exhaust gas cleaning apparatus for internal combustion engines or other combustion equipment is also provided, which comprises a natural radioactive element which emits 0.001 to 0.6 becquerels of .alpha.-rays by alpha-decay thereof. The radioactive element or a device including the same is disposed on the air inlet, exhaust gas outlet or fuel pipe of the equipment or engine to remove toxic components from the exhaust gas.