Investigation and study were conducted on spent vegetable oil in terms of its potential resource amount and adaptability to fuel when using it as petroleum substitution energy. Analysis of properties was made of spent soybean oil and rapeseed oil which form almost all the spent vegetable oil. As a result, it was found that spent vegetable oil is high in viscosity and distillation property and cannot be substituted for light oil in the way it is, but can meet standards for light oil if it is esterified. Moreover, spent vegetable oil cannot be used as fuel for gasoline-fueled automobiles, but can be used as a substitute of diesel powered automobile use light oil if esterified or mixed with light oil. For land internal combustion engine, it is usable if heated or mixed with light oil. For boiler use, spent vegetable oil can be used by itself, provided that heavy oil is used for firing and later spent vegetable oil takes the place of it. On the other hand, the resource amount of spent vegetable oil is below 1/100 of the demand for light oil, and substitutability of spent vegetable oil as resource is low. 83 refs., 63 figs., 72 tabs.