PURPOSE: To reduce an air surplus ratio at a high engine load operation so as to increase the output with a rich fuel air mixture provided by providing a valve which effects the closing motion of a fuel by-pass port linked with the throttle opening, coaxially with the valve shaft of a butterfly valve.

CONSTITUTION: A carburator 1 includes an air inlet 2 which receives clean air from an air cleaner, and a fuel inlet 3 which receives fuel gas, the pressure of which is reduced by a pressure regulator. A mixing section C where air is mixed with fuel, is formed in a ventury 4, a mixture from the mixing section C is fed to a combustion chamber of an internal combustion engine through a butterfly valve 7. In this case, a valve 6 is provided to the shaft end of the butterfly valve 7 in the carburator 1, and a by-pass 9 which is interrupted by the valve 6, is provided between a fuel passage B in the carburator 1 and a space D held by both the ventury 4 and the butterfly valve 7. The valve 6 includes a communication section 10 which is fully opened when the butterfly valve 7 is fully opened.