An electrolytic treatment apparatus and method by simply subjecting a chemical plating waste liquor containing phosphorus components and organic compounds to electrolytic treatment, thereby reducing the amount of impurities in the waste liquor to a level at which the waste liquor can be discharged. The chemical plating waste liquor containing hypophosphorous acid, phosphorous acid and organic compounds is subjected to electrolytic treatment in an electrolytic cell having a conductive diamond electrode as an anode. The conductive diamond electrode oxidizes or oxidatively decomposes the phosphorus components and organic compounds in the waste liquor substantially simultaneously, thereby reducing the amount of impurities to a level at which the waste liquor can be discharged.