PURPOSE: To improve response of air/fuel ratio correction and also ensure good atomization by a method wherein an injection orifice of an auxiliary fuel passage is provided at down stream of a throttle valve and an actuator for controlling a fuel rate on an auxiliary fuel passage is controlled in accordance with an output of an air/fuel ratio sensor.

CONSTITUTION: A carburetor 1 is equipped with a small venturi 9 with an opening of a main nozzle 10 communicated with a float chamber 11, and also a fuel passage between the float chamber 11 and a main nozzle 10, a main jet 12 and a main well 13 are incorporated. Furthermore, a slow passage 14 equipped with a slow jet 15 on the way between the main jet 12 and the main well 13 is incorporated. Yet more, an auxiliary fuel passage 6 communicated with a fuel jet orifice 4 installed at down stream of a throttle valve 2 is separately provided in communication with the float chamber 11, and an actuator 5 is attached to the jet orifice 4. This actuator 5 is controlled by a computer 8 in accordance with an output of an air/fuel ratio sensor 17 installed in an exhaust system to control the amount of fuel.