PROBLEM TO BE SOLVED: To provide a method of preparing pure (+)-1,4-dihydro-7-[(3S,4S)-3-methoxy-4-(methylamino)-1-pyrrolidinyl]-4-oxo-1-(2-thiazolyl)-1,8-naphthyridine-3-carboxylic acid (SNS-595).SOLUTION: A method of preparation comprises: (a) reacting SNS-595 with a first aqueous base, followed by neutralizing to obtain a primary hydrate; (b) dehydrating the primary hydrate and reacting the dehydrated product with a second aqueous base, followed by neutralizing to obtain a secondary hydrate; and (c) dehydrating the secondary hydrate to obtain a pure SNS-595 substance.SELECTED DRAWING: None【課題】純粋な(+)-1,4-ジヒドロ-7-[(3S,4S)-3-メトキシ-4-(メチルアミノ)-1-ピロリジニル]-4-オキソ-1-(2-チアゾリル)-1,8-ナフチリジン-3-カルボン酸(SNS-595)を製造する方法の提供。【解決手段】(a)SNS-595を第1の水性塩基と反応させ、次いで中和して、一次水和物を得ること、(b)一次水和物を脱水し、脱水された生成物を第2の水性塩基と反応させ、次いで中和して、二次水和物を得ること、(c)二次水和物を脱水して、純粋なSNS-595物質を得ることを含む、製造方法。【選択図】なし