PURPOSE: To prevent the fuel pulsation and at the same time, improve the control responsiveness by providing a negative pressure operated valve on the fuel control unit of a carburettor, and also a secondary supplying valve on the downstream side from a throttle valve, and then controlling fuel continuously by means of a negative pressure, while controlling the secondary air at two-position.

CONSTITUTION: Fuel to be supplied to the main nozzle 11 of a carburettor 3 is controlled through a control device 21 in response to the opening of a needle valve 12 in such a manner that a solenoid valve 16 for controlling negative pressure is controlled according to the various parameters 31W35 and the output of an O2 sensor 36, and then, a diaphragm valve 5 is driven by the negative pressure controlled herewith. As a result of fuel control by means of the needle valve 12, pulsation in the fuel does not take place. On the downstream side of a throttle valve 2 is provided a secondary air supply passage 40, and also provided a two-position control type solenoid valve 41 for opening and closing said passage, and then, the solenoid valve 41 is controlled through the control device 21, and consequently, a well-responded control can be performed.