To suppress adhesion of a foreign matter to a small metering part by forming a spiral groove in a metering part, to reduce production of a change of an air-fuel ratio of air-fuel mixture even after use for a long period, and to prevent an air-fuel ratio from being brought into a rich state.

Air entering through an inlet taper part 6 is brought into a swirl by a spiral groove 5 formed in the inner peripheral surface of a small part and stagnation does not occur. A large part 7 is formed in a spot situated downstream from the small part arranged in the spiral groove 5. Further, an air bleed jet 3 is formed in a cylindrical shape on a whole. This constitution suppresses adhesion of a foreign matter to the small metering part, reduces production of a change of an air-fuel ratio of air-fuel mixture, prevents the occurrence of stagnation of a flow of air, and prevents an air-fuel ratio from being brought into a rich state since adhesion of a foreign matter does not occur.