PURPOSE: To lower center of gravity of a vehicle by displacing sideward, among wire holders provided on a throttle drum, a wire holder which is arranged on an upper portion compared to the wire holder which is arranged on a lower portion, and thereby enabling arrangement of a main pipe on a lower portion.

CONSTITUTION: In a motorbike, a carburetor 23 is provided on an immediately lower portion of a main pipe of a vehicle frame. A float chamber 41 is formed on a lower part, while a throttle drum 44 is provided on an upper side surface. One end of a link mechanism, not shown, is installed on a center shaft 45 of the drum 44. The throttle drum 44 is formed by respectively integrating upper and lower wire holders 52, 53 to a cylindrical part rotatably supported to the center shaft 45. The upper wire holder 52 is displaced to the side of a vehicle body compared to the lower wire holder 53 in view of longitudinal direction of the vehicle. The upper wire holder is dislocated from an immediately lower portion of a main pipe 2, so that the main pipe 2 can be arranged on a further lower portion.