PURPOSE: To have pressure inside a cylinder balanced as well as to prevent fuel from leaking, by installing a seal part using an elastic body in two spots, top and bottom, of a push rod in an accelerator pump, while making supercharging pressure act on the intermediate part.

CONSTITUTION: An accelerator pump 1 is attached to the side of a charging cylinder 11 of a carburetor, while a piston 2 is housed inside its cylinder 3. Now, when stepping on an accelerator pedal, a shaft 5 and a lever 6 are rotated and this rotation is transmitted to a rod 7 and a lever 8 and thereby the piston 2 is pressed downward so that pressure in fuel 4 goes up, pushing a weight upward, thus the fuel is spouted into the charging cylinder 11 from an injector nozzle 10. In the above constitution, a bellows type seal member 12 and an annular seal member 13 directly contacting a push rod 15 are installed in two spots, top and bottom, of this push rod 15, through which an air passage 14 is made to interconnect the intermediate part, thereby making supercharging pressure act thereon, thus a fuel leakage is prevented from occurring.