To keep high hydrogen sulfide removal efficiency, economically remove hydrogen sulfide with a low consumption of a chemical agent, and simplify the constitution of a desulfurization apparatus.

Desulfurization of an object gas 2 to be treated is carried out by bringing the object gas 2 containing hydrogen sulfide into a gas-liquid contact tower 1, supplying a chemical liquid L produced by suspending an activated carbon powder 15 as a catalyst in an aqueous sodium hydroxide N solution from above the tower, upwardly bringing the object gas 2 into contact with the chemical liquid L, producing sodium thiosulfate by reaction of hydrogen sulfide of the object gas and sodium hydroxide N in the presence of oxygen in the ambient air in the tower or in the chemical liquid L and the catalyst, and absorbing the produced sodium thiosulfate in the chemical liquid L.