PURPOSE: To keep optimum idling speed always by connecting an opener to a throttle valve, forming a fuel by-path passage on the downstream side of the throttle valve and controlling the opener and the by-path passage according to load conditions and engine running conditions.

CONSTITUTION: A diaphragm type throttle opener 20 is connected to a throttle valve 4 through an interlocking mechanism 21. The throttle valve 20 is connected to an intake manifold 5 through a passage 22, and on the way to the passage 22, a three-way valve type solenoid valve 23 is installed. Moreover, a by-path passage 24 is branched from a slow system fuel passage 8 to connect it to the port 25 of the throttle valve 4 on the downstream side, and on the way to the by-path passage 24, a diaphragm type switching valve 26 is installed, which is connected to the intake manifold 5 through a passage 28 provided with a solenoid valve 27. The output signals of detecting means 30-35 for detecting respective load conditions, engine running conditions and so on are inputted into a control unit 40 for processing, thus energizing and controlling either or both of two solenoid valves 23, 27.