PURPOSE: A water-base washing or cutting oil including a fungicide is circulated while it is kept at a specific low temperature to prevent the growth of low-temperature resistant fungi in the oil used in metal cutting, lathing and car parts washing and extend the period when the oil is used by recycling.

CONSTITUTION: The water-base oil is sent back from the operation step to the relation or storage tank and cooled with a heat exchanger, preferably provided on the outside of the tank to keep lower than 25°C. At the same time, an antimicrobial that can control the growth of bacteria, fungi and yeasts is added to the oil by about 0.02, preferably 0.05%, thus enabling the long-term circulation of the oil. The preferably antimicrobial is, e.g., hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine or p-chloro-m-xylenol.