PURPOSE: To prevent a decline of combustion efficiency ascribable to an exhaust gas reflux by a method wherein a part of the exhaust gas is returned to a suction air passage through an oxygen pump within a reflux passage of the exhaust gas, and hydrogen generated through electrolysis of aqueous vapor remains in the exhaust gas.

CONSTITUTION: At the time of a heavy load operation of an engine, an exhaust gas reflux passage 11 is opened to reduce NOx in the exhaust gas, and a part of the exhaust gas farom an exhaust gas passage 10 passing through an oxygen pump 8 within the passage 11 is returned to a suction air passage 2. And, although direct current voltage is applied to an electrode of the oxygen pump 8, aqueous vapor in the exhaust gas is resolved by an electrolyte and oxygen is released into the air through an actuation of a control 9, a lowering of combustion efficiency is prevented through residual hydrogen in the exhaust gas.