An object of the present invention is to provide a drug and a method which suppress progress of disease in which the amount of amyloid β protein in the brain is increased, such as Alzheimer's disease. 1-(3-(2-(1-Benzothiophen-5-yl)ethoxy)propyl)azetidin-3-ol or a salt thereof has an effect of reducing the amount of amyloid β protein in the brain parenchyma, and thus is effective as an agent for reducing the amount of amyloid β protein in the brain. Disease in which the amount of amyloid β protein in the brain is increased, such as Alzheimer's disease, can be prevented or treated by administering 1-(3-(2-(1-benzothiophen-5-yl)ethoxy)propyl)azetidin-3-ol or a salt thereof.
