PURPOSE: To improve the drivability of acceleration, by driving a solenoid valve in an acceleration fuel passage to be controlled through a series circuit formed by an automatic switch, which is responsively actuated even in the time from whatever operative condition the accelerating operation is performed, and by an operation switch closed when a vehicle is in driving operation.

CONSTITUTION: Fuel in a fuel source 6 is pressurized by a pump P, and a device enables the fuel to be supplied into a float chamber 5 beside a carburetor main unit 1 through a fuel introducing path 7 and a fuel passage 8 while to an acceleration fuel nozzle 14 through an acceleration fuel passage 15 branching from and connecting with the fuel introducing path 7. A normally closed type solenoid valve S is provided interposing in the acceleration fuel passage 15, and said valve S is connected so as to form a series circuit with an operation switch H, opened when an engine is in operation, and an automatic switch T. And the automatic switch T is constituted providing a diaphragm 21 which acts in response to a difference between the pressure in the first receiving pressure chamber 22, introducing a negative pressure through a throttle member 29 and a check valve 30, and the pressure in the second receiving pressure chamber 23 directly introducing the negative pressure.