Abstract: A water treating apparatus comprising a plurality of treating tanks (2) connected together in series for removing contaminants from a liquid, and an electrode unit (6) disposed at the bottom of each treating tank (2). Water is passed sequentially from the most upstream treating tank toward the most downstream treating tank and is subjected to electrochemical treatment when flowing through the electrode unit (6) of each tank. The electrode unit (6) comprises a tubular case (61) made of an insulating material, and a plurality of electrode plates of a soluble metal each formed with a multiplicity of holes (82) and arranged within the case one above another at a spacing. Current is so passed that the adjacent electrode plates are different in polarity, with the uppermost electrode plate in each upstream treating tank differing in polarity from the lowermost electrode plate in the downstream treating tank adjacent thereto.