PURPOSE: To inhibit mixture from becoming lean in an engine, during high-opening low-speed operation and during accelerating operation and positively make the mixture rich in a specific operating region where the mixture becomes lean, in a fuel quantity increasing device for a carburetor.

CONSTITUTION: A fuel quantity increasing device for a carburetor is formed of a carburetor 1 provided with a throttle valve 9 for mechanically controlling the opening/ closing of the effective opening area of an intake passage 2, and a venturi part 11; a fuel inflow passage 17 for applying pressure to the fuel in a fuel tank T by a fuel pump P or the like so as to feed the fuel to a float chamber 5; a fuel quantity increase passage 20 with one end thereof connected to the fuel inflow passage 17 and with the other end thereof connected to the intake passage 2; and a control valve 21 for controlling the opening/closing of the fuel quantity increase passage 20 by a control signal from a control circuit 30 receiving the output of a throttle valve opening sensor 27 and a vehicle speed sensor 28. When the opening of the throttle valve 9 is the fixed value or more and the vehicle speed is within a range of the fixed vehicle speed determined in relation to the opening of the throttle valve 9, the control valve 21 is put in action by the control signal from the control circuit 30 to open the fuel quantity increase passage 20.