To certainly remove impurities from waste oil containing impurities to reuse the waste oil as regenerated oil.

The waste oil containing impurities is supplied into a prefilter 11 from a raw liquid tank 10 housing the waste oil containing impurites and the waste oil passed through the prefilter 11 is supplied to an electrostatic filter 15. A filter element having a large number of fine pores is incorporated in the prefilter 11 while an anode and a cathode are incorporated in the electrostatic filter 15 and the impurities in the waste oil passed through the prefilter 11 are removed by an electric field. A part of the waste oil cleaned by the prefilter 11 is returned to the raw liquid tank 10 and the waste oil in the raw liquid tank 10 is cleaned before it flows out of the raw liquid tank 10.