PURPOSE: To prevent any return flow of the intake air which is apt to occur at a low speed of an engine by disposing a reed valve and a throttle valve in a main suction passage and in an auxiliary suction passage, respectively to encourage swirl of sucked air at a low load run thereof.

CONSTITUTION: At a low load run of an engine with the first throttle valve 40 almost the fully closed position, a negative pressure which is detected by a load detecting port 76 is intense, on account of which a diaphragm 64 is pulled toward a negative pressure chamber 70 regardless of the speed of the engine so as to open the second throttle valve 60. Since a reed valve 56 is provided with the moderate trend of recovery, suction air passes through the auxiliary suction passage 42 at a low speed run to generate intense suction swirl inside a combustion chamber 16. At an intemediate load run, the negative pressure of suction air from a load detecting port 76 and Venturi negative pressure from a revolution seed detecting port 78 act together in the negative pressure chamber. At a high load run with the first throttle valve 40 almost in fully opened position, the negative pressure of suction air is weak, on account of which the second throttle valve 60 is opened and closed by the Venturi negative pressure.