PURPOSE: To prevent engine stop due to a sudden decelerating operation by constituting the interior of a carburetor so as not to supply over-rich mixture to an engine even at operating sudden deceleration.

CONSTITUTION: In a diaphragm type carburetor provided with a butterfly type throttle valve 17 and a mutually independent slow speed fuel system and a high speed fuel system, a slow speed fuel nozzle 20a on the uppermost stream side is opened on the way of the venturi part 16a of an intake passage 16 so that the negative pressure acting to the slow speed fuel nozzle 20a on the uppermost stream side of the intake passage 16 is stronger than that to slow speed fuel nozzles 20b, 20c on the lower stream side. At full load operation of an engine, air in the intake passage 16 is let flow into a slow speed fuel chamber 21. At suddenly decelerating the engine, the air in the slow speed fuel chamber 21 is sucked to the intake passage 16, and succeedingly the fuel in the slow speed fuel chamber 21 is sucked to the intake passage 16. As a result of this, fuel supply is momentarily delayed, over-rich of mixture is prevented, and hence engine stop at operating sudden deceleration is prevented.