PURPOSE: To restrain the mixture from becoming lean by operating a control valve in accordance with a control signal from a control circuit so as to open a fuel increase passage at an opening degree of a throttle valve higher than a predetermined value and in a range of vacuum in a venturi part.

CONSTITUTION: A plurality of carburetor bodies each formed therein with an intake-air passage 2 incorporating a throttle valve 9 and a venturi part 11. Further, a control valve 21 controls the quantity of fuel flowing through a fuel increase passage 20 in accordance with a control signal from the control circuit 30 which receives an output from a throttle valve opening degree sensor 27 and an output from a pressure sensor 28. Further, fuel increase distributing passages 20A to 20C are connected to the intake passages 2 of the carburetors, downstream of the control valve 21. At an opening degree of a throttle valve 9 and in a predetermined range of vacuum in the venturi part 11, the control valve 21 is operated in accordance with the control circuit so as to open the fuel increase passage 20. Thereby it is possible to restrain the mixture from rarefying.