PURPOSE: To prevent the danger accompanied by cleaning work due to the input in an oil tank by automatically cleaning the oil tank and to enhance the operation efficiency of the oil tank, by removing impurities contained in oil before the sedimentation thereof.

CONSTITUTION: Impurities (sludge) are removed before sedimentation while an oil tank 1 is used. That is, a recirculation pump 3, an automatic filter 4, a honeycomb mixer 5, a quantitative pump 6 and an oil heater 10 are respectively operated and a definite amount of an oil dispersant 7 is injected in the oil sent out from the oil tank 1 by the quantitative pump 6 and heated by an oil heater 10. Whereupon, the oil reaching the automatic filter 4 is made easy to receive filtering action and the automatic filter 4 automatically discharges sludge. The filtered oil is passed through the recirculation pump 5 and introduced into the upper part of the oil tank 1 by the honeycomb mixer 5.