To provide a purification process for fuel oil of good quality, emitting clean exhaustion gas, from inexpensive vegetable oil or vegetable waste oil in low costs, because of low installation costs.

A vegetable oil (virgin) or vegetable oil waste is converted to the fuel oil of diesel engine and purified by charging ozone water heated up to 40-80°C to the vegetable oil (virgin) or the vegetable oil waste heated at 40-80°C. Then, in the pretreatment process 2, an ozone gas of 4,000-30,000 ppm concentration are fed from the lower part under agitation to remove the impurities in the vegetable oil or waste oil, for example, glycerol, animal fatty acids or the like together with water, then the product that has been treated in the pretreatment process 2 is purified by charging an ozone gas of 4,000-30,000 ppm in the form of fine particles from the lower part of the treatment tank under agitation in the secondary treatment 5.