To sufficiently obtain dynamic pressure without inconvenience when an inner-vent type carburetor is arranged in an intake passage and sufficient dynamic pressure cannot be obtained in a position of an air vent intake port existing in the carburetor, and to achieve optimization of an air-fuel ratio.

An adapter component interposed between an intake side of the inner-vent type carburetor 4 and a connecting member 22 forming an upstream side intake pipe and varying the air intake port position of an air vent passage 40 is provided with a spacer part 30 having a center part penetrated by the intake passage 2 and extending the intake passage 2 for the thickness; and an extension pipe part 32 extending from an inner wall side of the spacer part 30 to a prescribed position in the intake passage 2, having a base end side connected to an air intake port 40a of the air vent passage 40 of the carburetor 4 and an end side opened to the upstream side in the intake passage 2, and extending the air vent passage 40 to the prescribed direction. The dynamic pressure of the intake air taken into the air vent passage 40 is adjusted by interposition.