PURPOSE: To improve a wide range of operability from a high rotation range to a low rotation range by providing a first actuator which opens a bypass circuit at the high rotation range and a second actuator which closes an air bleed at an opening time.

CONSTITUTION: Since intake air is introduced into an engine 2 through venturies 4,5 at low/middle speed ranges, the intake air is effectively throttled so s to maintain an air-fuel ratio of mixed air at a proper value. When the engine 2 is moved to a specified high operation range, a bypass passage 6 is opened by a first actuator 7. Since air is introduced into the engine 2 even from the bypass passage 6 a charging efficiency of the air at this range is improved. When the bypass passage 6 is opened, air bleed 8 of a carburetor 3 is closed by a second actuator 9. Fuel supplied from a nozzle 13 is increased according to bypass air quantity so as to maintain an air-fuel ratio at a stable value substantially.