This invention relates to a novel 4-oxo-1,4-dihydronicotinic acid derivative ##STR1## or a salt thereof, wherein R.sup.1 is hydrogen or carboxyl-protecting group;R.sup.2 is substituted phenyl and naphthyl, or a substituted or unsubstituted heterocyclic group; andR.sup.3 is haloalkyl, aminoalkyl, or substituted or unsubstituted alkenyl, phenylalkenyl, naphthylalkenyl, phenylalkyl, naphthylalkyl, phenylalkynyl, naphthylalkynyl, heterocyclic alkyl, heterocyclic alkenyl, phenyl, naphthyl, cycloalkyl, cycloalkenyl, carboxylic acyl, iminoalkyl, heterocyclic or bridged hydrocarbon, which has a broad antibacterial spectrum and a low toxicity, and are useful for treatment of diseases of human beings and animals, to a process for producing the same and to an antibacterial agent containing the same.