To improve a sterilization force, eliminate any bad effect to a work piece or a machine tool, and reduce load to a work environment or a nature environment, by adding a sterilization reinforcement composed of an alkaline metal salt of organic acid and an electrolyte for providing electric conductivity to an oil solution, and impressing direct current voltage on the oil solution to perform electrolysis of the sterilization reinforcement.

An alkaline metal salt of organic acid as a sterilization reinforcement is easily solved in water but has a weak sterilization action. Therefore, direct current voltage is impressed on an oil solution to which the sterilization reinforcement composed of the alkaline metal salt of organic acid and an electrolyte. Then, current flows through the oil solution to perform an electrolysis of the sterilization reinforcement, alkaline metal is deposited on an electrode to instantly become metal hydroxide. While, separated organic acid becomes 'acid' having strong sterilization action to provide excellent sterilization performance. By performing such sterilization method, off-flavor caused by putrefaction is prevented from occurring in an environment surrounding the oil solution, and the environment is kept clean. Accumulation of fungus products are prevented from causing degradation or alteration of the oil solution.