The invention provides a novel oxazolidinone derivative represented by the formula (I): wherein Ring A is optionally substituted or fused and represents (A-1) at least 7-membered monocyclic hetero ring containing at least three N atoms; (A-2) at least 6-membered monocyclic hetero ring containing at least two N atoms and at least one O atom; or (A-3) at least 7-membered monocyclic hetero ring containing at least two N atoms and at least one S atom; X1 is a single bond, —O—, —S—, —NR2—, —CO—, —CS—, —CONR3—, —NR4CO—, —SO2NR5—, and —NR6SO2— (wherein R2-R6 are independently hydrogen or lower alkyl), or lower alkylene or lower alkenylene in which one of the preceding groups may intervene; Ring B is optionally substituted carbocycle or optionally substituted heterocycle; R1 is hydrogen, or an organic residue which is able to bind to the 5-position of oxazolidinone ring in oxazolidinone antimicrobial agent, and an antibacterial agent containing the same.