PURPOSE: To prevent the damage of an oil cooler even if the discharge pressure of an oil pump is raised by providing a relief valve for bypassing the lubricant discharged from the pump through the cooler and an oil filter by operating the pump at the time of raise the discharge pressure of the pump.

CONSTITUTION: A lubricant is fully filled from the outlet of an oil pump 4 to an oil cooler 3 and an oil filter 5, supplied to the high speed bearing in a reduction gear 2, again sucked to the pump 4 and fed under pressure to the cooler 3. A relief valve 6 is provided from the inlet of the cooler 3 to the outlet of the filter 5, and the lubricant is fed to a lubricant line by bypassing the cooler 3 and the filter 5. Thus, even if the pressure loss in the cooler 3 is increased so that the discharge pressure of the pump 4 is raised, the damage of the pump 3 can be prevented.