PROBLEM TO BE SOLVED: To meet a requirement to establish a method of effectively reducing an acid value by an easy-to-use operation, the requirement brought about by such anxieties that a free fatty acid reduces biodiesel production efficiency, and reduces quality by a saponification substance in addition because the free fatty acid is generated by hydrolysis when using a cooking oil under a condition of high temperature.SOLUTION: Bentonite is added at a ratio of 0.1-50 pts.wt. to 100 pts.wt. of a cooking oil having an acid value of 10 mg KOH/g or less, and the mixture is stirred. When using aqueous bentonite, 150 pts.wt. or less of an amphiphilic solvent is further added to 100 pts.wt. of the cooking oil, and the mixture is stirred in order to enhance compatibility with the cooking oil. Thereby, the acid value of the cooking oil can be reduced to 5 mg KOH/g or less. A soap component is hard to generate while treating unlike a conventional acid value increase inhibitor or regenerant. Furthermore, the added bentonite and amphiphilic solvent can be separated from the cooking oil easily by suction filtration or centrifugal treatment.
