PURPOSE: To stably supply fuel, in the captioned carburetor having a constant oil level tank and returning tank, by operating a fuel controller through intake negative pressure of an engine and a quantity of intake air, feeding fuel to the constant oil level tank and controlling a supply quantity of the fuel with said fuel controller.

CONSTITUTION: At idle time, large intake negative pressure is guided into a chamber 15 through a passage 26, while small Venturi negative pressure is guided into a chamber 17 through a passage 28, and the second diaphragm 11 presses the first diaphragm 10, and is sucked to a side of the chamber 15. Accordingly, a valve body 18 of a fuel controller 7 decreases passage sectional area of a valve seat port 21, and a quantity of fuel, fed by a pump 6 to a constant oil level tank 4 through pipe lines 19, 22, can be reduced. While at full load high speed operation, the diaphragm 10, 11 are both moved downward, and a gap between the valve body 18 and the valve seat port 21 is increased, thus the quantity of fuel to the constant oil level tank 4 can be increased. Surplus fuel in the tank 4 is returned to a fuel tank 5 by a head through a reducing tank 3.