A method for electrochemical oxidation decomposition of difficult to decompose compounds such as aromatic compounds, organic chlorine compounds, agricultural chemicals, dioxin, PCB's and water soluble polymers that are contained in waste fluid, such as print developing waste fluids, factory waste water and industrial waste water, and landfill soil seeping waste water by carrying out electrolysis at a high current density to eliminate and purify difficult to decompose substances in the waste fluid. Waste fluid containing difficult to decompose substances is introduced into an electrolytic chamber having a ferrite electrode anode, electrolysis is performed at a high current density of 10 A/dm.sup.2 or higher, hypohalogenous acid and active oxygen are generated in the waste fluid along with a strong anode oxidation action being brought about, and the oxidation decomposition is repeated by re-circulating the waste fluid through a filter, electrolyzer and an intermediate collecting chamber.