To prevent a lean mixture at low speed operating time, when a carburetor temperature rises, while fuel flow amount characteristics are stabilized, by quickly sucking vaporized fuel generated in a fuel chamber to an intake passage and stabilizing pressure in the fuel chamber.

A first air vent passage 25 opened to the atmosphere and a second air vent passage 26 opened to an intake passage 3 are connected to a fuel chamber 10. In this way, during operation of an engine, a negative pressure generated in the intake passage is introduced to the fuel chamber 10 from the second air vent passage 26, vaporized fuel generated in the fuel chamber 10 is sucked from the second air vent passage 26, also the new air is introduced to the fuel chamber 10 from the first air vent passage 25, the fuel chamber 10 is scavenged, a pressure in the fuel chamber 10 is always stabilized, and a fuel flow amount characteristic is stabilized. Vaporized fuel generated in the fuel chamber 10 is quickly supplied to the engine, and a lean mixture at low speed rotation time according to a temperature rise of a carburetor is prevented.