24 grams of 5-(3-chloropropoxy)-3,4-dihydrocarbostyril and 17 g of sodium iodide are mixed with 300 ml of acetone and heated for 3 hours under refluxing conditions. Then, 300 ml of dimethylformamide, 12 g of triethylamine and 18 g of 4-benzylpiperazine are added thereto and the reaction is carried out at 60°-70° C. for 7 hours under stirring conditions. The reactions mixture is concentrated under a reduced pressure to obtain a viscous liquid, then 300 ml of 3% sodium hydrogencarbonate aqueous solution are added. The organic layer is extracted with chloroform and washed with water. After drying the chloroform layer, chloroform is removed by distillation. The residue thus obtained is washed with ether and recrystallized from methanol to obtain 32 g (yield: 84%) of 5-[3-(4-benzylpiperazinyl)propoxy]-3,4-dihydrocarbostyril in the form of colorless needle-like crystals with a melting point of 157°-159° C.
