PURPOSE: To improve the firing and borning performance and to purify the exhaust gas by performing the injection of air from a subsuction port in the low or middle load region where the frequency of usage is high while cutting or reducing it in quite low load or idling region or high load region.

CONSTITUTION: In an idling or low load region where a throttle valve 12 is fully closed or approximately fully closed, a throttle valve 18 in a subsuction path 17 is fully cosed or approximately fully closed to cut or limit the air from a subsuction port 14 to a combustion chamber 4 to small amount, thereby the leaning of mixed gas from a main suction port 5 is reduced to prevent the deterioration of firing and burning performance. While in the low or middle load region, the throttle valve 18 in the subsuction path 17 is interlocked with partial opening of the throttle valve 12 to open widely thus to inject the air into the combustion chamber 4 in spiral route along its inner wall face, thereby the firing performance of rich mixed gas is considerably improved while simultaneously the firing is stabled and the burning property is improved.