Ethyl 7-chloro-1-(2,4-difluorophenyl)-6-fluoro-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylate (I) is reacted with 3-aminopyrrolidine dihydrochloride (II), and then the resulting mixture is reacted with potassium hydroxide and p-toluenesulfonic acid monohydrate in order to obtain (rac)-7-(3-amino-1-pyrrolidinyl)-6-fluoro-1-(2,4-difluorophenyl)- 1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylic acid p-toluenesulfonate hydrate.