a novel carbostyril derivative and salt thereof represented by the formula ## str1 ## ), a group of the formula ## str3 ## ), 2 - methyl - 3 - nitrophenyl group , 3 , 5 - dichlorophenyl group , or a group of the formula ## str4 ## ); the carbon - carbon bond between 3 - and 4 - position in the carbostyril skeleton is a single or double bond ).