Prophylactic and/or therapeutic agent for mild cognitive impairment

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