To supply a lean air-fuel mixture satisfying both of the desired performance of an engine in a high opening area of a throttle valve, and the conformance of the amount of carbon monoxide to the exhaust emission control.

A back-pressure chamber 6 faced to a constant fuel chamber 5 through a diaphragm 7 is connected to an upstream side of the throttle valve 4 of an intake passage 2 through a negative pressure introducing passage 25, and the negative pressure on the upstream side of the throttle valve 4 is controlled by a fixed jet 26 mounted in the passage 25, a secondary-side space 21 of an air cleaner 18, and a manual control needle valve 31 mounted on a correction passage 27 connected to the passage 25, to be introduced to the back-pressure chamber 6. The lean air-fuel mixture can be obtained by tensionally displacing the diaphragm 7 by the controlled negative pressure.