PURPOSE: To prevent torsion of a rocking arm by positioning a differential gear provided for rear wheels on one side against the central axis longitudinal to a vehicle and a motor on counter side thereof.

CONSTITUTION: A rear axle 4 is mounted at rear end of a rocking arm 5, and front end of the arm 5 is coupled in free rocking motion to a frame 1a by a pivot 6 extending horizontally to a car body. A differential gear 8 is arranged on the rear axle 4 near to one side against the central axis 31 longitudinal to the car body 1. A DC motor 9 to be mounted on the differential gear is arranged on counter side of the differential gear 8 against the central axis 31. Weights of the differential gear 8 and the motor 9 are distributed left and right of the central axis 31 by this constitution, therefore torsion can be prevented from occurring due to force of inertia of these masses when the arm 5 rocks.