PURPOSE: To contrive stabilization of burning and an improvement of fuel consumption, by a method wherein suction air of high oxygen density is fed to an auxiliary chamber by providing an oxygen enrichment film on the midway of a suction duct.

CONSTITUTION: A suction duct 2 is ramified into a main chamber side suction manifold 4 connecting with a main combustion chamber 3 and an auxiliary chamber suction manifold 6 connecting with an auxiliary combustion chamber 5 in the midst of the suction duct 2. An oxygen enrichment film 17 is provided on a ramification part of the suction duct 2, sucked air of high oxygen density is fed to the auxiliary chamber 5 and sucked air of low oxygen density is fed to the main chamber 3. In this manner, stabilization of burning and an improvement of fuel consumption to be followed by an increase of a flame propagation speed can be contrived by securing of reduction of NOX generating within a combustion chamber.