Pyridone carboxylic acid compounds or physiologically hydrolyzable esters thereof are provided which are represented by the formula: ##STR1## wherein R.sub.1 is a lower alkyl, a halogen-substituted lower alkyl, a lower alkenyl, a cycloalkyl, or a substituted- or unsubstituted-phenyl group; R.sub.2 is a hydrogen atom, or a lower alkyl or an amino group; A is a nitrogen atom or the group C-X wherein X is a hydrogen or a halogen atom, or an alkoxy group; and Z is a group having the formula: ##STR2## wherein n is 1 or 2; R.sub.3 and R.sub.4 each represent a hydrogen atom or a lower alkyl group, with proviso that, if n is 2, one of R.sub.3 and R.sub.4 is a hydrogen atom; R.sub.5 and R.sub.6 each represent a hydrogen atom, or a hydroxy, a lower alkoxy or an amino group which is unsubstituted or substituted by a lower alkyl group, with proviso that one of R.sub.5 and R.sub.6 is a hydrogen atom; and R.sub.7 is a hydrogen atom or a lower alkyl group. The compounds of the present invention show potent and broad spectrum of antibacterial activities.