Abstract: The present disclosure relates to a use of a phloroglucinol-based compound and its salt for preventing and treating Alzheimer's disease. The pharmaceutical composition for preventing or treating Alzheimer's disease of the present disclosure fundamentally suppresses the cause of ?-amyloid formation, thereby exerting a remarkable effect of allowing ultimate treatment of Alzheimer's disease. The present disclosure provides the use of the single substance, which has been isolated from Dryopteris crassirhizoma and identified, for preventing or treating Alzheimer's disease for the first time.