To obtain 4-aminodiphenylamine at a low cost by hydrogenating 4-nitrosodiphenylamines in the presence of a hydrogenation catalyst, recovering the catalyst, and reusing the catalyst by hydrogen-treating the recovered catalyst in water and an organic solvent.

4-Nitrosodiphenylamines (e.g. 4-nitrosodiphenylamine) are hydrogenated by hydrogen gas having about 5-50 kg/cm2G pressure preferably in a solvent (e.g. a mixed solvent of water and benzene) in the presence of a hydrogenation catalyst (e.g. the catalyst obtained by allowing an activated carbon to carry about 1-5 wt.% Pd or Pt) preferably at about 0-100°C for about 0.5-10 hr, and the catalyst is recovered. The recovered catalyst is subjected to a hydrogen treatment in the water, an organic solvent (e.g. toluene) or a mixed solvent of the water and the organic solvent preferably at 0-200°C by the hydrogen gas having 0.1-100 kg/cm2G pressure for 0.5-10 hr to activate the hydrogenation catalyst in the method for activating the hydrogenation catalyst.