PURPOSE: To make a value of an air-fuel ratio of a gaseous mixture constant and proper by a method wherein a divergent ratio of a suction air flow into a small venturi is made constant by making a mean divergent angle at an outlet of the small venturi of a dual venturi type carburetor zero.

CONSTITUTION: An outlet of a small venturi 3 within an air suction duct 1 is provided so that the outlet can be opened at a throat of a large venturi 2 coaxially with the large venturi 2, a fuel nozzle 4 is made to open at the center of the small venturi 3 and the small venturi 3 is formed in parallel nozzle by making mean divergent angle of the outlet of the small venturi 3 zero. Because of the above, a variation of a divergent ratio to the small venturi 3 becomes smaller, and a negative pressure for drafting fuel generated in the small venturi 3 extending over all basin of a suction air flow in the air suction duct 1 becomes larger, then the air-fuel ratio of the gaseous mixture becomes about constant and proper.