To a solution of 50 ml of dimethylformamide with 5.0 g of 6-carboxy-3,4-dihydrocarbostyril and 4 ml of triethylamine, 3.87 g of isobutyl chloroformate in 2 ml of dimethylformamide solution was added dropwise. After stirring at a room temperature for 30 minutes, 5.5 g of benzylpiperazine in 3 ml of dimethylformamide solution was added to the former solution and stirred at a room temperature for 30 minutes, then stirred continuously at 50°-60° C. for 1 hour. The reaction mixture was poured into a voluminous amount of a saturated sodium chloride aqueous solution and extracted with chloroform and the chloroform extract was washed with water and dried. After removal of the solvent, to the residue thus obtained was added diethyl ether to crystallize the residue and recrystallized from ethanol to obtain 3.4 g of 6-(4-benzyl-1-piperazinylcarbonyl)-3,4-dihydrocarbostyril. Colorless needle-like crystals. Melting point: 198°-200° C.
