PURPOSE: To reliably control a ratio of a fuel injection volume to a wide-ranged suction volume, by divinding a middle part of a suction passage into a low speed suction passage and a high speed suction passage and by locating a high sensitive thermal line type suction volume detector at the low speed suction passage.

CONSTITUTION: A low speed suction passage 3 and a high speed suction passage 4 are located at a middle part of a suction passage 2 which has a high speed suction volume detector 9 at the upstream and a fuel injection valve 11 at the downstream, and stems of a low speed and a high speed throttle valves 6 and 7 located in the suction passages 3 and 4 are connected together through the medium of a link 8. In the case of a low speed suction, only a high sensitive thermal line type low speed suction volume detector 5 (having such a characteristic as illustrated in a solid line 16 in the diagram) located at the upstream in the low speed suction passage 3 works, and controls an injection volume of the fuel injection valve 11 via a control circuit 10. In the case of a high speed suction, the valves 6 and 7 open, a signal of only the high speed suction volume detector 9 (whose characteristic is illustrated in a solid line 17) is input to a control device.