PURPOSE: To control proportion of intake air passing a small venturi and to control an air-fuel ratio to an arbitrary value, an intake air passage in the outer periphery of the small venturi in which a main fuel nozzle is located is opened and closed by means of a control device.

CONSTITUTION: A main nozzle 3 is situated in a small venturi 2 situated to the central part of a large venturi 1, and a valve body 4a located to a screw 4 rotated with the aid of a motor 5 is disposed in an intake air passage formed in the outer periphery of the small venturi 2. When an output from an O2 sensor 8 is decided to be lean by means of a comparing circuit 7, a control device 6 drives the motor 5, and the screw 4 is closed in a range of 90°. Since the closing of the screw results in increase of an amount of air passing through the small venturi 2, a large amount of fuel is fed through a main nozzle for enriching. When an output from the O2 sensor 8 is a rich signal, the motion is reversed.