PURPOSE: To restrain the swing angle of drive wheels within a small range, by swingably supporting the casing of a power unit which carries drive wheels, to a vehicle body frame through a pivot shaft, and as well by attaching auxiliary units to the power unit on one side of the shaft opposite to the drive wheels.

CONSTITUTION: The casing 70 of a power unit P is rigidly formed such that the crank case 50 of an engine E is integrally incorporated with a pair of mission casings 71, 71' which extend rearward from the left and right sides of the crank case 50 to contain therein automatic transmission units M, M'. This casing 70 is swingably coupled to a vehicle body frame 4 by means of a pivot shaft 83, and as well, the drive wheels are supported to the casing 70 through a drive shaft 65. Auxiliary units such as, for example, a starter motor 24 and a water pump 45, are arranged on the front section of the engine E which is farthest from the drive wheels on one side of the above-mentioned pivot shaft 83, thereby, inertia moment around the pivot shaft 83 due to the drive wheels and the power unit may be made small.