20 Grams of 6-[4-(2-benzoylethyl)-1-piperazinylcarbonyl]-3,4-dihydrocarbostyril and 5.33 g of hydroxylamine hydrochloride were dispersed in 40 ml of pyridine and the suspension was stirred at 100° C. for 4 hours. After the reaction was completed, the reaction mixture was poured into water and then extracted with chloroform, the extract was dried with anhydrous potassium carbonate, then the solvent was removed by evaporation. The thus obtained residue was purified by means of a silica gel column chromatography (eluent: dichloromethane:methanol =50:1), then recrystallized from ethanol to yield 8.79 g of 6[-4-(3-hydroxyimino-3-phenylpropyl)-1-piperazinylcarbonyl]-3,4-dihydrocarbostyril 1/2-hydrate. White powdery substance. Melting point: 167° C.-169° C. (decomp.)
