The present invention relates to a novel quinolone carboxylic acid derivative of structural formula (I) and pharmaceutically acceptable salts thereof. These compounds also have a potent activity against both Gram-positive bacteria and Gram-negative bacteria; wherein R1 is hydrogen or alkyl group having one to six carbon atoms, R2 is alkyl group having one to four carbon atoms, haloalkyl group having one to four carbon atoms, cycloalkyl group having three to six carbon atoms, alkenyl group having two to four carbon atoms, or phenyl group which is optionally mono or disubstituted by fluorine atoms, X is hydrogen, amino, or halogen, Y is hydrogen or halogen, Z is (II) (wherein l is 1 or 2, m is 0, 1, or 2, and n is 0 or 1, R3 is hydrogen, alkyl group having one to four carbon atoms, haloalkyl group having one to three carbon atoms, acyl group, cycloalkyl group having three to six carbon atoms, aryl, or heteroaryl group).