Abstract: A method for removing nitrogen monoxide effectively from a nitrogen monoxide-containing gas which comprises contacting a nitrogen monoxide-containing gas with an aqueous solution containing a ferrous ion complex formed from a ferrous ion and an organic compound of 2 to 24 carbon atoms having in the molecule at least one atomic group selected from mercapto, amino, imino and nitrilo groups. When a sulfite or sulfide is present in the above aqueous solution, the nitrogen monoxide-absorbing capacity of the aqueous solution can be enhanced and nitrogen monoxide absorbed in the aqueous solution is reduced to non-toxic nitrogen. According to this method, nitrogen monoxide can be removed effectively, even if the gas to be treated contains an oxidizing substance such as oxygen.