PURPOSE: To reduce NOx in exhaust gas to harmless N2 with a cheap carbon source in the absence of catalyst by placing the source in an exhaust system and reducing the NOx with the carbon of the source.

CONSTITUTION: An internal combustion engine sucks a mixed gas of fuel and air into combustion chamber 5 demarcated by engine casing 3 and piston 4 from suction boat 2 which is opened or closed by suction valve 1, and it exhasuts exhaust gas toward exhaust duct 8 from exhaust boat 7 which is opened or closed by exhaust valve 6. Vessel 10 housing a great deal of granular solid carbon 9 is set to duct 8 so that it crosses the exhaust gas current flowing through duct 8. When the exhaust gas exhausted from boat 7 into duct 8 is passed through vessel 10, carbon 9 is heated to heat NOx in the gas, whereby the NOx are reduced to harmless N2 by the reducing action of a reaction product of carbon 9 and O2, i.e. CO. Unburned CO, hydrocarbon, etc. in the gas passed through vessel 10 are then oxidized in reactor 11 provided to the downstream side of vessel 10 to purify the gas.