To provide a quinolone antibacterial drug and a therapeutic agent for infectious diseases, which exhibit potent antibacterial activity on Gram-positive and Gram-negative bacteria and which is highly safe. A compound represented by the following formula (1): (wherein R 1 represents a C3-C6 cycloalkyl group which may have a substituent or the like R 2 represents a hydrogen atom or the like R 3 and R 4 each independently represent a hydrogen atom or a C1-C6 alkyl group, or a substituted carboxyl group derived from an amino acid, a dipeptide, or a tripeptide, and, in the case where each of R 3 and R 4 represents a C1-C6 alkyl group, the alkyl group may be substituted by one or more atoms or groups selected from among a hydroxyl group, a halogen atom, a C1-C6 alkylthio group, and a C1-C6 alkoxy group and n denotes an integer of 1 to 3), a salt thereof, and a hydrate of the compound or the salt. Also, antibacterial drugs and therapeutic agents for infectious diseases are prepared.
