PURPOSE: To reduce resistance to the fluid flow at the time when a throttle valve is fully opened and to achieve fine control of the throttle-valve opening at the time of low-speed operation of an engine, by forming recesses in the inner peripheral surface of an air supply passage of the engine at the portions thereof located near the throttle valve.

CONSTITUTION: A pair of recesses 17, 17 are formed in the inner peripheral surface of an air supply passage 2 of a carburetor 1 at the portions thereof located near a throttle valve 12 and on the opposite side of the direction in which turning ends of the throttle valve 12 are turned from the completely closed position of the valve 12. Each of these recesses 17, 17 has a deepest portion 18 at the position near the turning ends of the valve 12 when it is located at its completely closed position. By employing such an arrangement, it is enabled to reduce resistance to the fluid flow by increasing the sectional area of the air supply passage when the valve 12 is fully opened and to reduce the sectional area of the passage at the time of starting opening motion of the valve 12 in low-speed operation of an engine, without adverse influences caused by the recesses.