A heterocyclic compound having a structure in which a benzene ring and a nitrogen-containing heterocycle are condensed is added to a sulfuric acid solution that contains a persulfuric acid component such as peroxomonosulfuric acid, peroxomonosulfate, peroxodisulfuric acid, or peroxodisulfate as an oxidant and in which impurities that promote the reduction in the oxidant concentration are present. Here, the heterocyclic compound having a structure in which a benzene ring and a nitrogen-containing heterocycle are condensed is preferably a benzotriazole-based compound. The impurities that promote a reduction in the oxidant concentration are suitably one or more selected from copper ions, iron ions, nitrate ions, and nitrite ions. The method of the present invention of inhibiting a reduction in the oxidant concentration in a sulfuric acid solution containing a persulfuric acid component can inhibit the reduction in the oxidant concentration to a minimum even when impurities that cause a reduction in the oxidant concentration are mixed.