In a power unit comprising an engine (13) and an HST (24) which are formed into an integral body, a lubricating oil for use in the engine and an operating oil of the HST (24) are commonized. After being cooled, the common oil is fed from the side of the engine (13) to the HST (24). Such arrangement suppresses the drop in viscosity of the common oil and prevents the drop in efficiency and the occurrence of seizing on the side of the HST (24).