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 quinolone 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.