PURPOSE: To reduce an air-fuel ratio when feed air pressure is abnormally high and to smoothly reduce the number of revolutions of an engine, by installing an air control valve which increases the amount of the air bled when the feed air pressure of a surge tank exceeds given pressure.

CONSTITUTION: When feed air pressure within a surge tank 3 exceeds a set value, feed air pressure flows in a chamber 6a of an air control valve 6 through a passage 13 and is exerted on a flexible film member 8. As a result, member 7 is released from adhesion to a seat plate 11 against a spring 9, and the air flows in a chamber 6b through an air filter 12. Thereafter, the air flowing in the chamber 6b flows in an air introducing passage 4 of a carbretter 1 through an air passage 14. This enables reducing of an air ratio, and permits the smooth reduction of the number of revolutions of an engine when feed air pressure increases during high rotation.