Abstract: Carbostyril derivative represented by the formula (I): ##STR1## wherein R.sup.1 and R.sup.2, which may be the same or different, each represents a hydrogen atom, a cycloalkylcarbonyl group, a cycloalkylalkanoyl group, a benzoyl group, an alkanoyl group or a phenylalkylcarbonyl group; R.sup.4 represents a hydrogen atom or an alkyl group, R.sup.5 and R.sup.6, which may be the same or different, each represents a hydrogen atom, an alkyl group, a cycloalkyl group, or a phenylalkyl group, or R.sup.5 and R.sup.6 may, when taken together with the nitrogen atom to which they are attached, form a 5- or 6-membered substituted or unsubstituted heterocyclic ring containing 1 or 2 nitrogen or oxygen atoms as hetero atoms; and the ring A has a partial structure: ##STR2## wherein R.sup.3 represents a hydrogen atom, a cycloalkylcarbonyl group, a cycloalkylalkanoyl group, a benzoyl group, an alkanoyl group or a phenylalkylcarbonyl group, with the proviso that at least one of R.sup.1, R.sup.2 and R.sup.3 when R.sub.