PURPOSE: To provide a cutting oil feeder that is composed so as to prevent any deterioration of the performance of cutting oil and decomposition.

CONSTITUTION: A coolant stock solution feeder 7, a water feeder 8, a hydrogen ion concentration improver feeder 9 and a preservative feeder 10 are all set up in a tank 3 for cutting oil 11, and further a cutting oil concentration measuring instrument 5 and a hydrogen ion concentration measuring instrument 6 are installed in this tank 3, and moreover, there is provided a control unit 2 which is connected to these measuring instruments 5, 6 and controls these feeders 7, 8, 9 and 10 on the basis of the measured results.