At least one of ammonia and ethylenediamine is reacted with monoethanolamine in the presence of a catalyst containing phosphoric acid or its condensate in order to prepare acyclic ethyleneamines, and the catalyst can be improved in performance and the life of the catalyst can be prolonged by feeding a phosphorus-containing material to a reaction system during the reaction. In addition, the catalyst having a deteriorated performance can be reactivated by adding the phosphorus-containing material to the catalyst.