In order to insure that, even when there arises a change in the rate of air flowing through an intake mixture passageway, the air-fuel ratio of the mixture which is to be supplied to an engine is always kept constant, the fuel control system comprises: an intake mixture passageway having a first negative pressure generating section and a second negative pressure generating section provided upstream of the first negative pressure generating section for generating a negative pressure weaker than that in the first negative pressure generating section; a fuel passageway having its one end opening in the first negative pressure generating section and its other end connected to a float chamber via fuel metering jet; an electromagnetic valve for controlling the rate of the fuel flowing through the fuel passageway; a negative pressure passageway having its one end opening in the second negative pressure generating section and having its other end connected to the fuel passageway at a site located between the fuel metering jet and the electromagnetic valve; and a level detecting means for detecting whether or not the fuel column formed within the negative pressure passageway is at a preset level and capable of generating an electric signal to control the operation of the electromagnetic valve. The first and second negative pressure generating sections are each constructed by a fixed and/or a variable venturi.