PURPOSE: To increase the size of a clutch with correspondence to increase of engine output by arranging the axis of a starting clutch above the line connecting between the axis of crank shaft and the axis of rear wheel.

CONSTITUTION: A starting clutch 23 is provided on the end 22b at the opposite side from the end 22 at the wheel fixing side of a drive shaft 22 coupled through a gear 21 to the axle 20 of the rear wheel 5 or on the end 22b at the opposite side from the engine 7. Here the axis A of said clutch 23 or the axis of the drive shaft 22 is arranged above a line D-D connecting between the axis B of a crank shaft 10 and the axis C of the axle 20 of the rear wheel 5. When inclining the chassis 1, the angle for a belt power transmission system 4 to reach to the ground is made wider than that for the chassis cover 1a to contact with the ground. Consequently even when the size of said clutch 23 is enlarged with correspondence to increase of engine output, said clutch 23 is protected from contacting against the ground and damaged.