PURPOSE: To promote atomization of fuel in a low speed area by providing a Venturi tube on an upper steam of a throttle valve in a primary suction passage on a lower stream of a supercharger and opening and closing the secondary valve in accordance with the difference between a pressure of an upper stream part of the throttle valve on the lower stream of the supercharger and a pressure of the Venturi part.

CONSTITUTION: The primary and the secondary suction passages 3 and 4 with throttle valves 5 and 6 are provided in a suction chamber 2 at the lower stream of a supercharger and a Venturi tube 25 is provided on the upper stream side of the throttle valve 5 in the primary suction passage 3. A pressure chamber 22 of an actuator 7 connected with the secondary throttle valve 6 is connected with the Venturi tube 25 and the pressure chamber 23 is connected with the upper stream side of the secondary throttle valve 6 to open and close the secondary throttle valve 6 by the pressure difference between both pressure chambers. In this way, the secondary throttle valve can be opened and closed only in connection with the flow speed change of the Venturi neck part (the primary throttle valve opening degree and the revolution number) regardless of the supercharging pressure, so that the high speed air flow can be obtained in the primary suction passage at low speed to promote atomization of fuel.