A fuel supply device for carburetors comprising a fuel passageway (10) opened at one end in a variable venturi (4) and connected to a float chamber (13) through a jet (12); a solenoid valve (14) and/or fuel pump (15) provided within the fuel passageway; a negative pressure passageway (16) opened at one end in a stationary venturi (9) located on the upstream side of the variable venturi and generating a negative pressure weaker than in the variable venturi and having an upright part connected to the fuel passageway between the jet and the fuel pump; a level sensor (17) provided in a predetermined height position of the upright part of the negative pressure passageway and capable of detecting whether or not a fuel column formed in the upright part is in a predetermined position and; a control circuit capable of controlling the operation of the solenoid valve and fuel pump on the basis of the output from the level sensor in order to make it possible to maintain the air-fuel ratio in a high load range to be proper without enlarging the flow resistance within the suction bore so that the operation of the fuel pump and solenoid valve may be controlled by the level sensor and control circuit so that the fuel column may maintain the predetermined height.