6,7-Difluoro-1-cyclopropyl-8-methyl-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (1.8 g) is suspended in N-methylpyrrolidone (5 ml) and thereto is added piperazine (1.8 g), and the mixture is stirred at 150° C. for 3 hours. After the reaction, the reaction mixture is concentrated, and the resulting residue is purified by silica gel column chromatography (solvent, dichloromethane:methanol=3:1) to give 7-(1-piperazinyl)-1-cyclopropyl-6-fluoro-8-methyl-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (0.06 g), as white crystals, m.p. 291°-295° C. (decomp.).
