PURPOSE: To prevent the scattering in air-fuel ratios through analogically controlling air-fuel ratios by providing the fuel control unit of a carburettor with a valve operated under negative pressures, and then controlling said negative pressures by means of an electronic control device in response to the operating state of an engine and the output of an O2 sensor.

CONSTITUTION: An auxiliary jet 8 is provided by bypassing a main jet 6 which leads to the main nozzle 11 of a carburettor 3, and needle valve 12 is arranged on the auxiliary jet 8. The needle valve 12 is operated by means of a diaphragm 5 in response to negative pressures in a negative pressure chamber 13. The negative pressure in the negative pressure chamber 13 controls the opening and closing of a solenoid valve 16 through a control device 21 so that a prescribed air-fuel ratio is obtained according to the various parameters 31W35 of engine and the output of an O2 sensor 36. The solenoid valve 16 controls the negative pressure in the negative pressure chamber 13 by way of opening and closing a negative pressure pore 17 so that the pressure ratio of the negative pressure to the atmospheric air from an atmospheric pressure pore 18 is changed, and performs a continuous fuel-supply quantity control.