PURPOSE: To reduce a combustion pressure and reduce the striking noise of gears by slightly opening an open/close valve which is placed in a high load suction passage when an engine is idling.

CONSTITUTION: A low load suction passage 11 and a high load suction passage 12 are laid independently from each other in the vicinity of a suction port 9 which opens to a combustion chamber 7 of an engine. In idle running, with a low load throttle valve 22 almost fully closed, negative pressures from primary and secondary negative pressure take-out openings 17 and 18 are led into a negative pressure chamber 16d of a diaphragm device 16. An open/close valve 13 in the high load suction passage 12 is thereby opened to a minimal degree to decelerate the velocity of a suction air which flows into the combustion chamber 7, hence easing a swirl therein. Since a combustion pressure is reduced and the fluctuation of an angular speed of a crank is thereby moderated, a striking noise of gears may be reduced.