PURPOSE: To compact the width of an engine, by a method wherein a V-belt type continuously variable speed change gear is disposed in the direction of the width of a car and internally of a primary reduction means and at the rear of a crank shaft.

CONSTITUTION: In a drive unit 2, when the rotation speed of a crank shaft 5 is increased and a centrifugal clutch 7 is actuated, rotation of the crank shaft 5 is reduced by means of a primary reduction means 19, proper speed change is effected by means of a V-belt type continuously variable speed change gear 18, and after that, rotation is transmitted to a rear wheel through an intermediate shaft 17 and an output shaft 22. Since the V-belt type continuously variable speed change gear 18 is disposed inside the primary reduction means 19 and at the rear of a crank chamber 6c, i.e. approximately to a central part in the direction of width, an input shaft 11 and an output shaft 12 are prevented from outward protrusion from the crank shaft 5, and enables sharp compacting of the width of an engine.