Abstract: When two kinds of ions are dissolved in a solution, the permeation of these ions through an ion exchange membrane is not equivalent each other. Further, an ion exchange membrane passes ionic species but it does not pass neutral species. These phenomena are termed “the permselectivety” and make it possible to separate both species or extract some species dissolving in a solution. Separation is specific performance of an ion exchange membrane and it is applied widely in many fields. Drinking water should not dissolve excessive components. Particularly, toxic substances should be excluded from drinking water. The quality of drinking water is prescribed by the guidelines for drinking water by the governments in every country. WHO‟s guidelines are the international standard setting for drinking water and a part of them are presented in Table 13. 1. ED (Electrodialyzer) is available to regulate the quality of drinking water according to the guidelines.