A power unit has a power case which includes a crankcase portion integrally formed with a transmission case portion. The crankcase portion forms a crankcase in which the crankshaft of a 4-cycle internal combustion engine is received. The transmission case portion forms a transmission case in which a wet type multiple disc friction clutch for transmitting the torque of the crankshaft is received. A partition wall makes the crankcase independent of the transmission case, and thus separates the lubrication system of the internal combustion engine from the lubrication system of the friction clutch. The lubrication structure of the present invention can improve the durability of the friction clutch and make the friction clutch compact, and secure good connection performance of the friction clutch by separating the lubrication system of an internal combustion engine from the lubrication system of friction clutch.