PURPOSE: To achieve the combusting condition with NOx generation well suppressed, by furnishing a pressurizing pump and an O2-enriched film on a divergent path which diverges from a suction path, and supplying an O2 rich air to a main chamber and an N2 rich air to an aux. chamber.

CONSTITUTION: A engine concerned is equipped with an aux. chamber, in which a divisional valve 8 is arranged at a communication hole 3 leading from this auxiliary chamber 2 to a main chamber 1. A pressurizing pump 16 is installed on a divergent path 23 which diverges from a suction path 19 downstream the compressor 14 of a turbo-charger 12, and a separate film 15 as an O2- enriched film is provided downstream of this pump 16 while a selector valve 29 is arranged at the point of suction path 19 where the divergent path 23 diverges. At the separate film 15, this divergent path 23 diverges into an O2 rich path 24 leading to a suction port 10 for the main chamber 1 and an N2 rich path 18 leading to the auxiliary chamber 2. An auxiliary chamber valve 19 is installed on this N2 rich path 18 which opens to the auxiliary chamber 2, while a suction pump 17 is mounted on the O2 rich path 24 with its downstream end connected to the suction path 19.