1. A prophylactic and / or therapeutic agent for mild cognitive impairment, where the agent contains, as an active ingredient, a carbostyril derivative expressed by the general formula (1) or a salt thereof: wherein A represents an unbranched chain alkylene group having a carbon number of 1 or more to 6, R represents a cycloalkylaryl group having a carbon number of 3 to 8, and the bond between positions 3 and 4 of the carbostyril skeleton is a single bond or double bond. 2. The prophylactic and / or therapeutic agent for mild cognitive impairment according to claim 1, wherein the carbostyril derivative expressed by the general formula (1) is 6- [4- (1-cyclohexyl-1H-tetrazol-5-yl) butoxy] -3 4-dihydrocarbostyril. 3. The prophylactic and / or therapeutic agent for mild cognitive impairment according to claim 2, wherein the agent is administered to a group of patients with a MMSE score of greater than or equal to 22 and less than or equal to 26.4. The prophylactic and / or therapeutic agent for mild cognitive impairment according to claim 2 or 3, wherein the powder is 6- [4- (1-cyclohexyl-1H-tetrazol-5-yl) butoxy] -3,4-dihydrocarbostyril with an average particle size, less than or equal to 4 microns, mixed as an active ingredient in a surfactant. 5. An interstitial flow improver comprising, as an active ingredient, a carbostyril derivative expressed by the general formula (1) or a salt thereof: wherein A is a straight chain alkylene group having 1 to 6 carbon atoms, R is a cycloalkylaryl group, having a number of carbon atoms from 3 to 8, and a bond between positions 3 and 4 of the carbostyril skeleton1. Профилактическое и/или терапевтическое средство для умеренного когнитивного нарушения, где средство содержит в качестве активного ингредиента производное карбостирила, выраженное общей формулой (1), или его соль:где A представляет собой алкиленовую группу с неразветвленной цепью, имеющую число атомов углерода от 1 до 6, R представляет собой циклоалкиларильную группу, и
