PURPOSE: To enable the automatic operation of a working machine by receiving an oil feed signal out of the working machine, charging and mixing a solute and a solvent at a specified rate automatically and producing the cutting oil.

CONSTITUTION: A level of cutting oil left behind in a mixing tank 1 is always measured by a level switch 49, and when this switch 49 shows less than the lower level, if an oil feed signal is issued from a working machine, a solute charging valve 25 comes to ON, and a metering cup 27 is tilted by receiving a solute from this valve 25. This tilting value is measured by a metering check switch 39, and when inclination of more than the specified quantity is measured within the specified time, the solute is changed in the mixing tank 1 by a charging unit. In addition, at this time, a solvent is charged into the mixing tank by a solvent charging valve 45 as well. This charging is performed till the level switch 49 shows its upper level. With this constitution, specified quantities of the solute and the solvent are mixed in the tank 1 to produce the cutting oil.