Ethyl 7-(4-acetyl-1-piperazinyl)-6-fluoro-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylate (7.2 g) and anhydrous potassium carbonate (4.1 g) are added to dimethylformamide (60 ml). The mixture is heated at 50° C. for 30 minutes with stirring. 2-Fluoroethyl p-toluenesulfonate (6.5 g) is added to the mixture, and the mixture is heated at 70°-80° C. with stirring. After the reaction is completed, the insoluble substance is removed by filtration. The filtrate is concentrated to dryness under reduced pressure, and the precipitate is collected by filtration. The precipitate is recrystallized from acetone to give 6.3 g of ethyl 7-(4-acetyl-1-piperazinyl)-6-fluoro-1-(2-fluoroethyl)-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylate (m.p. 183°-185° C.).
