A QUINOLONE SYNTHETIC ANTIBACTERIAL AGENT HAVING EXCELLENT PROPERTIES AS A MEDICINE IS PROVIDED, WHICH HAS STRONG ANTIBACTERIAL ACTIVITY NOT ONLY TO GRAM-NEGATIVE BACTERIA BUT ALSO TO GRAM-POSITIVE COCCI THAT HAVE LOW SENSITIVITY TO QUINOLONE ANTIBACTERIAL AGENTS, AND WHICH EXHIBITS HIGH SAFETY AND EXCELLENT PHARMACOKINETICS. A COMPOUND REPRESENTED BY THE FORMULA (I) OR A SALT THEREOF, OR A HYDRATE THEREOF. SPECIFICALLY, A QUINCLONE DERIVATIVE OF THE FORMULA (I) WHEREIN SUBSTITUENTS R6 AND R7 TAKEN TOGETHER WITH THE CARBON ATOMS TO WHICH THEY ARE BONDED FORM A CYCLIC STRUCTURE WHICH MAY CONTAIN AN OXYGEN ATOM AS A RING CONSTITUENT ATOM, THE CYCLIC STRUCTURE FORMING A 5- 4, 5-5, OR 5-6 FUSED BICYCLIC PYRROLIDINYL SUBSTITUENT, THE SUBSTITUENT BEING BONDED TO A QUINOLONE MOTHER SKELETON Q CONTAINING A PYRIDOBENZOXAZINE STRUCTURE:
