Provided is a prophylactic and/or therapeutic agent for mild cognitive impairment, which can improve interstitial flow in cerebral blood vessels and the like to achieve the sufficient clearance of harmful proteins accumulated in the brain. The prophylactic and/or therapeutic agent comprises 6-[4-(1-cyclohexyl-1H-tetrazol-5-yl)butoxy]3,4- dihydrocarbostyril or a salt thereof as an active ingredient. The prophylactic and/or therapeutic agent can improve the flow of an interstitial fluid around blood vessels in a drainage pathway to excrete harmful proteins. The prophylactic and/or therapeutic agent for mild cognitive impairment according to the present invention may take the form of a preparation for oral administration, a liquid preparation for oral administration, or an injectable preparation. The prophylactic and/or therapeutic agent for mild cognitive impairment according to the present invention can be used for the prevention and/or treatment of cerebral amyloid angiopathy, and can also be used for the amelioration of dementia with Lewy bodies, Down's syndrome or macular degeneration.
