Abstract: A hydraulic motor and a hydraulic pump are interconnected by a closed hydraulic circuit. A lubricating chamber is defined around the sliding surfaces of the swash plate and the plungers of at least one of the hydraulic pump and the hydraulic motor. A hydraulically hermetic chamber is defined in surrounding relations to at least one of the hydraulic pump and the hydraulic motor. The hydraulically hermetic chamber is connected to a replenshing pump by a relief valve which is openable at a pressure level. A pressure control valve is disposed in a discharge passageway of the hydraulically hermetic chamber and openable at a first pressure level lower than the pressure level for the relief valve. In another embodiment, a second pressure control valve is connected to a junction between the first pressure control valve and the lubricating chamber and openable at a second pressure level lower than the second pressure level.