A power transmission mechanism of a motorcycle includes a drive case which is disposed at the rear end of and outside of a rear swing arm, and which surrounds the axle of a rear wheel and incorporates a transmission device. In addition, the power transmission mechanism includes a drive shaft extending from an output shaft of an engine for transmitting power of the drive shaft to the rear wheel. The transmission device converts the direction of a driving force of the drive shaft and transmits it to the rear wheel. The drive shaft constituting part of the power transmission mechanism is disposed below the rear swing arm and the pivot shaft. The result is a motorcycle configuration that suppresses the vehicle width without expanding the rear swing arm.