A shaft of a pinion gear is engaged with a ring gear extends toward the front side in a final gear case that is provided in a power transmission system between a power unit and a wheel. Splines are provided on each of an outer circumference of the shaft, and on an inner circumference of a cylindrical portion of a pinion joint that forms a receiving portion of a joint provided at the rear end of a drive shaft for transmitting power from the power unit to the pinion gear. The two splines are engaged with each other. The pinion joint is fixed to the pinion gear by a bolt that is threadedly engaged with a threaded portion formed at the lower portion of a hole in the pinion gear shaft. A flexible portion of the bolt is formed between a head portion and the threaded portion of the bolt.