A dimethyl sulfoxide (15 ml) solution containing 5-acetylamino-6,7-difluoro-1-[(1R,2S)-2-fluoro-1-cyclopropyl]-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid (3.0 g, 8.47 mmol), (3S,4R)-4-(1-tert-butoxycarbonylaminocyclopropyl)-3-fluoropyrrolidine (3.19 g, 12.7 mmol) and 1-methylpiperidine (1.54 ml, 12.7 mmol) was heated at an outer temperature of 80° C. for 22 hours under a condition of 2.94×108 Pa. The reaction solution was washed with ethyl acetate to adjust the total volume to 128 ml and then washed twice with 5% citric acid aqueous solution (30 ml) to separate the organic phase and aqueous phase. The aqueous phase was extracted three times with ethyl acetate, combined with the above organic phase and then extracted three times with 5% potassium hydroxide aqueous solution (30 ml). To the aqueous phase were added chloroform (90 ml) and then 3 N hydrochloric acid until pH became 4, thereby separating the organic phase and aqueous phase. The aqueous phase was extracted twice with chloroform, combined with the above organic phase and then dried over anhydrous sodium sulfate. By evaporating the solvent under a reduced pressure, 5-acetylamino-7-[(3S,4R)-4-(1-tert-butoxycarbonylaminocyclopropyl)-3-fluoropyrrolidinyl]-6-fluoro-1-[(1R,2S)-2-fluoro-1-cyclopropyl]-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid (4.92 g, purity 90.0%, yield 89.2%) was obtained as yellow amorphous.
