A compound represented by general formula (I), a medicinally acceptable salt or ester thereof, a process for producing the same, and an antibacterial containing the same as the active ingredient: wherein R1 represents hydrogen or lower alkyl; R2 represents hydrogen or a negative charge; R3 represents hydrogen or lower alkyl; Ar represents lower alkyl, lower alkylsulfamoyl, etc. (each of which may be substituted by hydroxyl, di (lower alkyl) sulfonyl, etc.), or phenyl, naphthyl or a group of formula α or β (each of which may be substituted by hydroxyl, di (lower alkyl) sulfamoyl, etc.), wherein A¿4? and A5 represent each a single bond, -NHSO2-, etc., and Het represents pyrrolinyl, 1,4-diazabicyclo[2.2.2]octanyl, etc. (each of which may be substituted by hydroxyl, carbamoylated lower alkyl, etc.); A1, A2 and A3 represent each a single bond or lower alkylene which may be substituted by lower alkyl, lower alkylsulfamoyl, etc. (each of which may be substituted by hydroxyl, di(lower alkyl)sulfamoyl, etc.) or may be substituted by pyridyl, pyridinio, etc. (each of which may be substituted by lower alkyl, carbamoylated lower alkyl, etc.); and W represents sulfur, a single bond, etc.