A motorcycle is provided in which a seat and a storage box are arranged so as to be vertically stacked on each other in a rear portion of a body frame, and a pivotally mounted powertrain unit is suspended from the body frame under the storage box. The powertrain unit extends in the fore-and-aft direction of the body frame, and the engine has a crank chamber and a lubricant oil chamber provided separately from each other. A lubrication-oil passage structure for the engine is provided between each of a pair of journal bearing portions of a crankcase. The crankcase has an enclosed crank chamber and a crankshaft which is freely rotatably supported by the journal bearing portions with plain bearings. The journal bearing portions are provided with protrusion bosses for resisting entry of lubricating oil into the crank chamber from outside of the crankcase.