PURPOSE: To maintain and control a metal working oil without generating any bad smell, by mesuring the oxidation-reduction potential of the metal working oil, and sterilizing the oil properly on the basis of the found progress of the putrefaction thereof.

CONSTITUTION: The oxidation-reduction potential of a metal working oil is measured, and a germicide, e.g. hexahydro-1,3,5-tris-(2-hydroxyethyl)-s-triazine or 5-chloro-2-methyl-4-isothiazolin-3-one, is added or the oil is heat-treated at 50W60°C until the potential becomes 0W+50mV (a stage of generating no bad smell at 104 germs reducing sulfuric acid/ml). Thus, the oxidation-reduction potential is recovered to about ≥+100mV.

EFFECT: The sign of the generation of a bad smell can be easily sensed in a short time.