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 5 clearance of harmful proteins accumulated in the brain. The prophylactic and/or therapeutic drug includes 6-[4-( 1-cyclohexyl-l I l-tetra/ole-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 drainage a pathway to excrete harmful proteins. The prophylactic and/or therapeutic drug for mild cognitive impairment may take the form of 10 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. 48
