The present invention provides a 4-oxoquinolizine antibacterial agent having a 2-pyridone skeleton as a partial structure and also having a strong antibacterial effect on gram-positive bacteria, gram-negative bacteria or anaerobic bacteria. The compound having the following formula (I) or a pharmaceutically acceptable salt thereof: ##STR00001## wherein: R.sub.1 represents hydrogen atom or a carboxyl-protecting group, R.sub.2 represents hydrogen atom, a halogen atom, a lower alkyl group, a lower alkoxyl group or hydroxyl group, R.sub.3 represents phenyl group or an aromatic substituent selected from the group consisting of 5-membered and 6-membered heterocyclic groups and R.sub.3 has a substituent selected from the group consisting of a hydrogen atom, a lower alkyl group, a lower alkoxyl group, a nitro group, a cyano group, an amino group, an acyl group, a carbamoyl group and a ureido group, and R.sub.4 represents a hydrogen atom or a halogen atom.