PURPOSE: To remove an impurity at high efficiency by arranging a highly dielectric material with a smooth surface between a pair of electrodes whose opposing faces are coated with an electrical insulating material and contacting it with liquid oil with application of voltage.

CONSTITUTION: Liquid oil contg. an impurity in a tank 1 is sent to a heater 3 by a pump P and, after heated to a predetermined temp., supplied to an electrostatic treater 4. A large number of highly dielectric materials 8 with a smooth surface are arranged in a clearance 7 between a pair of opposing electrodes 5 and 6 whose opposing faces are coated with an electric insulating material and, after an impurity is removed from the oil by applying A.C. voltage of 5W10kV/ cm2 between the both electrodes from an A.C. source 9, the oil is introduced into an electrostatic settling tank 10 having a coalescer C to remove a liquid impurity and then filtered through a microfilter F provided at the top inside the vessel 10 to be recovered to a receiver R.