To supply fuel to an intake air passage from a long bag-shaped fuel pump at starting time and accelerating time of an engine by using intake pressure of the engine or pulsing pressure of a crankcase.

The pulsing pressure (the intake pressure of a 4-stroke internal combustion engine) of the crankcase of a 2-stroke engine is made to act on an outer peripheral surface of the long bag-shaped fuel pump 34 composed of a restorable material such as rubber built in a carburetor body 2 via a through hole 27a penetrating through a valve stem 27 of a butterfly throttle valve 28. The fuel inside the fuel pump 34 is supplied to a passage 35a between a high speed fuel adjusting needle valve 29 and a check valve 32. A manual opening-closing valve C is arranged for opening and closing the through hole 27a in an idle position of the throttle valve 28, and the fuel of the fuel pump 34 is supplied to the intake air passage 14 via the check valve 32 by opening the opening-closing valve C when starting the engine. The through hole 27a opens the opening-closing valve C in a fully opening position of the throttle valve 28.