The present invention addresses the problem of providing a pharmaceutical agent and a method for suppressing the progression of diseases such as Alzheimer-type dementia indicating an increase in the level of amyloid-β protein in the brain. 1-(3-(2-(1-benzothiophen-5-yl)ethoxy)propyl)azetidin-3-ol or a salt thereof has an effect of decreasing the level of amyloid-β protein in the brain parenchyma, and hence is effective as an agent for decreasing the level of amyloid-β protein in the brain. By administering 1-(3-(2-(1-benzothiophen-5-yl)ethoxy)propyl)azetidin-3-ol or a salt thereof, it is possible to prevent or treat diseases in which an increase in the level of amyloid-β protein in the brain occurs, such as Alzheimer-type dementia.
