Abstract: Waste water to be treated is introduced into a first electrolytic bath, and then, hydroxide iron particles are supplied into the waste water through the electrolysis reaction at iron electrodes. Impurities such as organic compounds, phosphoric compounds, persistent compounds and the like are physically absorbed onto the hydroxide iron particles. Then, the hydroxide iron particles (flocks) with absorbed impurities are discharged via a drain valve, to purify the waste water to some degree. The waste water is introduced into a second electrolytic bath with a water pump. In the second electrolytic bath, the remnant impurities in the waste water are chemically dissolved and removed through the oxidation reduction reaction between the surfaces of metallic oxide electrodes and the waste water.