In order to know the electrolytic decomposition characteristics of ammonia to nitrogen, this work has studied an adsorption and a electrolytic reaction of ammonia at an IrO₂ anode with a change of pH, and has evaluated the decomposition rate to nitrogen, the change of byproducts, and the effect of chloride ion on the decomposition reaction. Also, an electrolytic decomposition mechanism of ammonia was suggested. The ammonia in the basic solution was oxidized mainly to nitrogen gas through a direct electrolytic reaction accompanying adsorption at the electrode where the oxygen evolution hardly occurred. The ammonium ion in the neutral and acid solution could be partly decomposed to nitrogen by the OH radicals generated in the condition of oxygen evolution, not by the direct electrode reaction. However, the decomposition rate in the neutral or acid solution was much lower than that in the basic condition. The hypochlorite ion generated from chloride ion by the electrode was very effective for the decomposition of ammonia and ammonium ion to nitrogen. The ammonium ion by hypochlorite in neutral condition was partly changed to NOx gas as well as to nitrogen gas.