The novel carbostyril derivative represented by the general formula (1) ##STR1## [in the formula, A, R.sup.1, R.sup.2 and the carbon-carbon bond between 3- and 4-positions in the carbostyril skeleton have the same definitions as above] has an excellent platelet aggregation inhibitory activity, a phosphodiesterase inhibitory activity, a venticular contraction activity (a positive contraction activity), an anti-ulcer activity, an anti-inflammatory activity, a cerebral blood flow increasing activity, a thrombus dissociation activity, a thromboxane A.sub.2 antagonism activity, etc. The carbostyril derivative of the present invention is characterized in that it exhibits the above activities for a long time, is low in toxicity (particularly low in toxicity to hearts of cardiovascular hyperplasia, myocardipathy, etc.), and is low in circulation activities such as heart rate increasing activity, blood pressure decreasing activity and the like. Also, the carbostyril derivative of the present invention has advantages in that it is easily absorbed by the intestinal tract and easily migrates into blood. Therefore, the carbostyril derivative of the present invention can be used most suitably as a preventive and remedy for thrombi such as apoplexia, cerebral infarction, myocardial infarction and the like, a cerebral circulation improving agent, an anti-inflammatory agent, an anti-asthmatic agent, a cardiotonic agent and a phosphodiesterase inhibitory agent.