A compound of the formula: ##STR1## wherein R.sup.1 is a hydrogen atom or a lower alkyl group, R.sup.2 is a hydrogen atom or a negative charge, R.sup.3 is a hydrogen atom or a lower alkyl group, Ar is a phenyl group, a naphthyl group or a group of: ##STR2## (wherein each of A.sub.4 and A.sub.5 is a single bond, --NHSO.sub.2 -- or the like, and Het is a pyrrolinyl group, a 1,4-diazabicyclo�2.2.2!octanyl group or the like which may be substituted with a hydroxyl group, a carbamoyl lower alkyl group or the like) which may be substituted with a lower alkyl group, a lower alkylsulfamoyl group or the like which may be substituted with a hydroxyl group, a di-lower alkylsulfonyl group or the like; a hydroxyl group; a di-lower alkylsulfamoyl group or the like, each of A.sub.1, A.sub.2 and A.sub.3 is a single bond or a lower alkylene group which may be substituted with a lower alkyl group, a lower alkylsulfamoyl group or the like which may be substituted with a hydroxyl group, a di-lower alkylsulfamoyl group or the like; a pyridyl group or a pyridino group; and W is a sulfur atom, a single bond; or a pharmaceutically acceptable salt or ester; a process for its production and an antibacterial agent containing it as an active ingredient.