PURPOSE: To inexpensively obtain a purified oil having low contents of water, sulfur compound, oxygen, nitrogen compound and sludge by installing a gas- liquid contact device, a condenser, an oil-water separator and a water removing device.

CONSTITUTION: Waste oils occurring from automobile maintenance factories, gas stations, etc., are collected, heated under about 300-500Torr reduced pressure and treated with steam to carry out gas-liquid contact. Then, gas part subjected to steam distillation is condensed to give liquid part, from which water is separated, the residual oil is furthered treated by a centrifugal separator and the remaining water is removed to give the objective purified oil having ≤0.3wt.% water content and ≤0.5wt.% ash content.