TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Catalyst: The amount of alkali catalyst is adjusted such that the content of free fatty acid in raw material oil is 3 mass% or less. The alkali catalyst is potassium hydroxide.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Processes: Water and odorant are distilled off from the raw material oil with the free fatty acid. Solid content, water, salts and free fatty acids are removed from the raw material oil by gravity separation and centrifugation before distilling the raw material oil. The raw material oil is transesterified using a catalyst-containing alcohol solution obtained by dissolving an alkali catalyst in a lower alcohol. The generated fatty acid ester is mixed with the raw material oil. Excess alcohol is depressurizingly distilled from the transesterified reaction liquid mixture. Gravity separation of the reaction product is then carried out, and the light liquid is extracted and mixed with an absorption agent. The light liquid is then processed by filtration and/or by gravity separation. Dilute sulfuric acid with a predetermined amount of water is added to the transesterified reaction liquid mixture, and the liquid mixture is neutralized. Sulfate crystals are separated by filtration from the neutralized reaction liquid mixture, and filtrate is distilled. The high density glycerol compound is then separated by gravity separation. The amount of water is 1-20 mol equivalent w.r.t the amount of sulfate.Preferred Component: The lower alcohol is methyl alcohol. The amount of lower alcohol is 50 mass parts or less w.r.t the total amount of the mixed solution, to adjust the viscosity of the mixed solution.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Catalyst: The amount of alkali catalyst is adjusted such that the content of free fatty acid in raw material oil is 3 mass% or less. The alkali catalyst is potassium hydroxide. EXAMPLE - Raw material oil having an acid value of 10, an iodine number of 105, a flash point of 230degreesC, a water content of 5 mass% and an ash content of 2.5 mass%, was taken in a raw material oil storage tank and heated to 95degreesC. The raw material oil was further heated to 145degreesC and poured in a vacuum deoxidation tower at an absolute pressure of 0.8 mmHg for 15 minutes. The raw material oil was cooled to 65degreesC. Potassium hydroxide (11 mass parts) dissolved in methyl alcohol (100 mass parts) having a purity of 99.5% was mixed with the raw material oil and reacted for 15 minutes, at an absolute pressure of 100 mmHg at 65degreesC. An alkyl ester layer (light liquid) and glycerol layer (heavy liquid) of the obtained product were separated. Solid substance was isolated from the light liquid and filtered, to obtain bio-diesel fuel. The yield of alkyl ester w.r.t fatty acid glyceride in the raw material oil was 99.5%.