5-Amino-7-((3S,4S)-3-amino-4-methyl (or ethyl)-1-pyrrolidinyl)-1-cyclopropyl-6-fluoro-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid or a pharmacologically acceptable salt thereof represented by the following formula wherein asymmetric carbon atoms marked with asterisks are in the S-configurations and R1 representsmethyl group or ethyl group; and an antibacterial agent comprising said compound as an active ingredient. <IMG>
