Abstract: Antibacterial quinolone derivatives represented by the following formula and salts thereof are disclosed. ##STR1## wherein R.sup.1 meawns a substituted or unsubstituted lower alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted lower alkenyl group or a substituted or unsubstituted phenyl group, R.sup.2 denotes a hydrogen atom or a carboxyl-protecting group, R.sup.3 represents a hydrogen or halogen atom or an amino, mono- or di-(lower alkyl)amino, hydroxyl or lower alkoxyl group, X is a hydrogen or halogen atom, Y means a nitrogen atom or a group C-R.sup.4 in which R.sup.4 is a hydrogen or halogen atom or a lower alkyl or lower alkoxyl group or is a group ##STR2## which forms a ring together with R.sup.1 (i.e., the asterisked carbon atoms are linked to the N(1) atom of the quinolone skeleton), and A denotes a specific N-containing group. Preparation processes of the quinolone derivatives and antibacterial agents containing the same are also disclosed.