Glycerin is effectively produced from a waste oil by a method comprising adding a strongly alkaline reducing water obtained by electrolyzing water containing sodium chloride to a waste oil selected from the group consisting of a waste edible oil, a fossil fuel waste oil and a mixture thereof, and extracting natural glycerin or synthetic glycerin by chemical reaction. Further, reusable edible oil is produced from a waste edible oil by a method comprising electrolyzing an aqueous solution having an electrolyte in an electrolytic cell, mixing a strongly alkaline reducing water generated at a cathode side of the electrolytic cell with a waste edible oil under ordinary temperature condition, and conducting oil/water separation, thereby purifying the waste edible oil to an extent such that it can be edible.