To easily and perfectly detoxicate harmful compds. by electrochemical decomposition by dissolving harmful compds. contained in the material to be treated into an aprotic solvent and executing energizing by the plus oxidation potential or minus reduction potential in an electrolytic cell.

Various harmful organic compds. are preferably once extracted with an organic solvent, are concentrated and dissolved into an aprotic solvent such as acetonitrile or the like. This solution is added with perchlorate as supporting salt to obtain an electrolytic soln. This electrolytic soln. 20 is stored into an electrolytic cell such as a double layer type electrolytic cell 15 in which the electrolytic soln. 20 is divided by a diaphragm 14, or an electrolytic cell of a three layer system, the one with no diaphragms or the like, at the electrolytic cell 18 on the side of the anode, a platinum electrode 11 and an Ag/Ag+ reference electrode 12 as working poles are arranged, and, at the electrolytic cell 19 on the side of the cathode, a carbon platinum electrode 13 as the counter pole is arranged. On the space between the electrodes, energizing by the oxidation potential of about ≥+1.5V or the reduction potential of about ≤2.0 V is executed, and electrochemical decomposition is executed. In this way, the harmful organic chemical substance is decomposed and is converted into the perfectly harmless substance.