FIG. 3 shows the ionic state when an ion exchanger 62 a mounted on a processing electrode 64 and an ion exchanger 62 b mounted on a feeding electrode 66 are brought into contact with or close to a surface of a workpiece 60, while a voltage is applied via a power source 67 between the processing electrode 64 and the feeding electrode 66, and a liquid 68, e.g. ultrapure water, is supplied from a liq