PURPOSE: To provide a carburettor which adjusts the internal pressure of a float chamber by introducing the pressure in the suction flow-path into the float chamber, wherein provision of a suction pressure lead-in pipe likely to ill influence the flow of the suction gas is eliminated.

CONSTITUTION: A rotary shaft 9 of a choke valve 3 installed in a suction flow- path 1 is formed in a pipe whose one end is blocked, and an opening 10 is formed at the peripheral surface of the rotary shaft 9 which is directed to the air flow-in side when the choke is opened, and an air vent 12 furnished in the upper part of a float chamber 5 is connected with the blocked end of the rotary shaft 9 through an air vent tube 13, and thereby it is made practicable to put the float chamber 5 in communication with the suction flow-path 1.