Data on a secular change of each denitration catalyst is managed based on data obtained by a periodic maintenance and a daily management. Management of a secular change and prediction on performance variations that occur until a next periodic check is performed. It is determined whether the denitration catalyst is deteriorated such that an exhaust-gas denitration system cannot maintain its performance. When the denitration catalyst is deteriorated, regeneration, replacement, or addition of the denitration catalyst is performed, and the denitration catalyst is altered as necessary. When the denitration catalyst is usable, the denitration catalyst is not replaced nor regenerated.