PURPOSE: To adsorb and collect the oxidized deteriorated substances and solid impurites contained in oil to be treated, by providing a heating part for heating the oil to be treated and a dehydration treatment part.

CONSTITUTION: The oil to be treated received in an oil tank is pressurized by a pump 1 to be supplied to a clay receiving part 3 where the oxidized deteriorated substances in said oil are adsorbed. Then, the oil to be treated passes through a filter 4 to remove the solid impurities in the oil and is subsequently sprayed into a vacuum-resistant oil container 6 from a nozzle 6a. The oil to be treated is heated by a heater 6b in the vacuum-resistant oil container 6 and the moisture in the oil is removed by evaporation. Next, the oil to be treated is supplied to the oil tank 2 by a pump 8 and again pressurized by the pump 1 to be supplied to the clay receiving tank 3. At this time, clay is heated by the oil to be treated in the clay receiving part 3 and the moisture adsorbed in the clay is forcibly discharged into the oil to be treated.