PURPOSE: To let mixed cutting oil be uniform in concentration by providing a mixing tank in which measured stock solution is mixed with water so as to be stirred, and the mixture is diluted by a definite ratio while being mixed to produce cutting oil.

CONSTITUTION: A stock solution tank 1 storing stock solution for cutting oil is provided. A measuring tank 2 is provided, which measures a definite amount of stock solution supplied from the stock solution tank 1 by the use of means detecting for level, volume, weight and the like. And next, an amount of water corresponding to measured stock solution is supplied, and both the solution and water are stirred and mixed so as to be diluted by a definite ratio, so that mixed cutting oil is thereby produced in a mixing tank 3. In order to smoothly perform cutting work such as cutting, grinding and the like, mixed cutting oil which is so produced that stock solution is diluted so as to be automatically mixed, is automatically or manually supplied to process machines.