A power unit of a vehicle, which includes an internal combustion engine and a transmission, is provided which is configured to quickly return lubricating oil fed to the transmission to an oil storage portion of the power unit case. The transmission includes a gear train housed in a gear chamber, the gear train executing power transmission among an input shaft, an intermediate shaft and an output shaft. The input shaft and the output shaft are arranged such that a line connecting their respective shaft centers extends almost horizontally above a communicating port which permits return oil flow between segregated portions of the power unit case. In addition, the intermediate shaft is disposed above and between the input shaft and the output shaft.