The present invention is a method of producing a 6-fluoro-7-substituted quinolonecarboxylic acid derivative represented by the general formula (4): ##STR1## wherein the variables are defined in the specification, which is characterized by adding an alkylboric acid ester represented by the general formula (3):B (OR.sub.5).sub.3 (3)wherein R.sub.5 represents a lower alkyl group, an aryl group, a substituted aryl group or a substituted aralkyl group to a 6-fluoroquinolonecarboxylic acid derivative represented by the general formula (1): ##STR2## wherein R.sub.1 and R.sub.2 are as defined in the specification; and R.sub.3 represents an optional, functional residue capable of participating in a nucleophilic substitution reaction, and a substituted saturated heterocyclic amine represented by the general formula (2): ##STR3## wherein X, Y and n are as defined in the specification, and condensing the reaction mixture with heating.