To provide a decomposing method of DXNs in a waste gas, which is capable of obtaining a high DXNs decomposition efficiency even at a low temperature, reducing the quantity of a catalyst to be used for attaining practical performance and suppressing the effect of SOx at the minimum.

The chlorine-containing organic compound is oxidation decomposed by nitrogen dioxide by bringing nitrogen dioxide contained in a waste gas or added into the waste gas into contact with the chlorims-containing organic compound contained in the waste gas in the presence of a catalyst consisting essentially of titanium oxide, molybdenum oxide and vanadium oxide in the Ti/Mo/V atomic ratio of (99-70)/(0.5-15)/(0.5-15) at 100-450°C.