Abstract: A method of producing biodiesel fuel from fats and oils includes a step of transesterification reaction in which fats and oils are transesterfied with lower alcohol, a step of fuel refining in which a resultant fatty acid alkyl ester produced in said transesterfication reaction step is refined, and a mixing step in which the fatty acid alkyl ester refined in the fuel refining step is mixed with light oil. The refined fatty acid alkyl ester is heated to a predetermined temperature and thus fused so as not to oxidize unsaturated fatty acids contained in the fatty acid alkyl ester, while the heated and refined fatty acid alkyl ester is mixed with light oil so as not to set the temperature of the mixed solution below the cold filter plugging point during the mixture, to dilute the fatty acid alkyl ester by light oil to a predetermined weight ratio.