PURPOSE: To remove fine impurities, completely prevent the putrefaction and odor of water soluble cooling oil following filtration and sterilization and improve products in surface precision by arranging oil degradation preventing filtering material partition walls.

CONSTITUTION: Three oil degradation preventing filtering material partition walls 4 are provided in a cooling oil reservoir 3 in a cooling oil tank 2 furnished with a cooling oil return passage 1. Cooling oil circulated and returned through the cooling oil return passage 1 is entered into the first cooling oil reservoir 3, passed through the degradation preventing filtering material partition walls 4 and transferred to a cooling oil supply pump 5. In the meantime, impurities mixed into the cooling oil re filtrated, trapped and sterilized to make the cooling oil clean, which is fed via a cooling oil circulation pump, connected to nozzle 10 and entered into the cooling oil tank 2 through the cooling oil return passage 1 again, on a repetitive basis.