PURPOSE: To control the quality of the cutting fluid automatically by a method wherein the pH value or the quantity of H2S is detected by a detector for the supervision of properties of the cutting fluid and the signal from the detector is outputted to a fluid sterilizing section and the fluid is regenerated while maintaining its original properties.

CONSTITUTION: Emulcut B-50, with an antiseptic added to it beforehand in a raw cutting fluid tank 1 and diluted with water to a predetermined concentration in an automatic diluting device 2, is supplied through a pipe 19 to a coolant tank 16 and then pressure-fed to a cutting machine through a pipe 21 by a pump 15. A signal is produced by the detector 4 when the pH value of the cutting fluid lowers below the set point and inputted to a dicision circuit 5. By this signal, a heating steam valve 7 is opened and a steam is supplied to a heat exchanger 9 consisting of a part of the sterilizing section and or the other hand, by said signal, a valve 10 is opened to stir up the fluid within the tank 16. After one hour, the supply of steam is stopped by a timer 6 and a motor 13 started for driving by the actuation of a timer 11 to operate an oil skimmer to skim the oil and regenerate the fluid.