PURPOSE: To obtain a method for producing 4,4'-diaminostilbene-2,2'-disulfonic acid or its salt of stable quality without forming any by-products such as red colored substances even by repeatedly using a catalyst.

CONSTITUTION: This method for producing 4,4'-diaminostilbene-2,2'-disulfonic acid or its salt comprises continuously charging an aqueous solution or an aqueous suspension of 4,4'-dinitrostilbene-2,2'-disulfonic acid or its salt in the presence of a platinum-active carbon catalyst in an aqueous solution at pH5-8 and a temperature within the range of 50-150°C while keeping the reactional pressure of hydrogenation under ≥1kg/cm2 (gauge pressure) and catalytically hydrogenating the 4,4'-dinitrostilbene-2,2'-disulfonic acid or its salt in the liquid phase.