A 5-aminoquinolone carboxylic acid derivative represented by the following general formula (1): ##STR1## wherein R.sup.1 means a hydrogen atom, an amino group, a linear or branched alkyl group having 1-5 carbon atoms and an alkylamino group containing a linear or branched alkyl group having 1-5 carbon atoms, R.sup.2 denotes a hydrogen atom or a linear or branched alkyl group having 1-5 carbon atoms, X stands for a halogen atom, Y means CH.sub.2, NH, CHR.sup.3 NR.sup.3, (R.sup.3 denoting a linear or branched alkyl group having 1-5 carbon atoms) or an oxygen atom, and Z stands for an oxygen atom or 2 hydrogen atoms, or a salt thereof, and an antibacterial agent containing such a derivative or salt as an active ingredient.The derivative or salt thereof exhibits strong anti-bacterial activities against Grampositive bacteria including tolerant bacteria, and is satisfactorily absorbed in a living body.