A nitrogen-containing heterocyclic compound represented by the general formula: wherein the dashed line represents a single bond or a double bond; R 1 , R 2 , R 3 , R 4 and R 5 independently represent a hydrogen atom, halogen atom, a lower alkyl, aryl, lower alkoxy or monocyclic heterocyclic group which may be substituted or the like; R 6 represents a lower alkyl, aryl, monocyclic heterocyclic, bicyclic heterocyclic or tricyclic heterocyclic group which may be substituted; X 1 represents a lower alkylene group or the like; X 2 represents a lower alkylene, lower alkenylene or lower alkynylene group which may be substituted; X 3 represents an oxygen atom, sulfur atom, a sulfinyl group, sulfonyl group or the like; Y 1 represents a bivalent cyclic group, containing a nitrogen, which may be substituted or the like; and Z 1 represents a nitrogen atom, a carbon atom which may be substituted or the like, or a salt thereof. The compound or salt has a potent antibacterial activity and a high safety, and is therefore useful as an excellent antibacterial agent.