Biodiesel as fatty acid methyl esters is an alternative for fossil diesel that is commercially produced by transesterification of oils with methanol under vigorous stirring with an alkaline catalyst. This is because oils and methanol cannot be miscible each other to have two phase reaction system. For further dissemination, waste oils and unused plant oils with toxic substances should be efficiently used as biodiesel. However, the alkali-catalyzed method is not simply applicable to the lower-grade waste oils, because such oils contain not only triglycerides as major component but also free fatty acids as minor component, the latter of which reacts with alkaline catalyst to produce saponified products. Therefore, non-catalytic supercritical methanol methods such as one-step supercritical methanol method (Saka Process) and two-step supercritical methanol method (Saka-Dadan Process) have been developed. By these methods, transesterification of triglycerides and esterification of free fatty acids proceed simultaneously without any catalysts, because the ionic products of methanol is increased in its supercritical condition, while a dielectric constant of methanol is decreased such that triglycerides and methanol can become miscible each other in its supercritical state and its reactivity can be tremendously enhanced. After completion of its reaction, however, fatty acid methyl esters as biodiesel can be phase-separated from methanol in an ordinary condition. Thus, pure biodiesel can be readily achieved. Such attracting behaviors of supercritical fluid technology were discussed to overcome problems in two-phase reaction system in the alkali-catalyzed method.