PURPOSE: To aim at stabilizing an idling speed and increasing the maximum output, by installing an auxiliary air passage, which bypasses a Venturi tube and opens to the upstream side of a throttle valve, in a suction passage as well as installing an on-off valve which opens the auxiliary air passage with such suction pressure as being higher than the constant one lying between the Venturi tube and the throttle valve.

CONSTITUTION: A Venturi tube 3 and a throttle valve 4 are installed in a suction passage 2, while an auxiliary air passage 5 bypassing the Venturi tube 3 and being open to the upstream side of the throttle valve 4 is installed in the suction passage 2 in parallel. And, air flowing into the suction passage 2 by way of an air cleaner 6 is made ro branch off to the Venturi tube 3 and the auxiliary air passage 5, then joined together again and fed to an engine through the throttle valve 4. At this time, a downstream end face of the auxiliary air passge 5 is blocked up by an on-off valve 7. With this constitution, in time of engine driving, in proportion as the opening of the throttle valve 4 grows large, suction pressure in a part between the Venturi tube 3 and the throttle valve 4 goes up, thus the on-off valve 7 is opened and the air from the auxiliary air passage 5 is eventually made to be supplied.