An object of the present invention is to provide a drug which prevents progress of Alzheimer's disease and a method of preventing progress of Alzheimer's disease. Alzheimer's disease can be effectively prevented or treated by initiating administration of a pharmaceutical composition comprising 1-(3-(2-(1-benzothiophen-5-yl)ethoxy)propyl)azetidin-3-ol or a salt thereof as an active ingredient to a patient within a certain period from clinical diagnosis of Alzheimer's disease.
