To reduce the fuel quantity and at intermediate speed so as to comply with exhaust gas regulation and, at the same time, discharge a needed fuel flow rate at high speed so as not to lower output.

A fuel flow control passage 76, whose one end is opened in a venturi of a suction passage 12 (an opening part 74) and the other end is connected between a check valve 70 and a main jet 66 in a main fuel passage 54, is provided. This connects an opening part 74 of the fuel flow control passage 76 with a main nozzle 56 of the main fuel passage 54. Within the range of intermediate speed, the position of the main nozzle 56 is greater than that of the opening part 74 in negative pressure, so that fuel containing air, which is introduced from the opening part 74 to the main fuel passage 54 through the fuel flow control passage 76, is delivered from the main nozzle 56 to the suction passage 12, thereby reducing a fuel flow in the range of intermediate speed as compared to a previous case. Within the range of high speed, the position of the main nozzle 56 is equal to that of the opening part 74 in negative pressure, and the fuel is discharged from both the main nozzle 56 and the opening part 74 to the suction passage 12, and thereby a fuel flow rate from which a required output is obtained is ensured.