The present invention contemplates provision of a quinolone antibacterial agent and a drug for the treatment of infectious diseases, which exhibit potent antibacterial activity and higher selective toxicity against Gram-positive and Gram-negative bacteria, which do not cause side effects (e.g., convulsion), and which exhibit higher safety. A compound represented by the following formula (I): a salt of the compound, or a hydrate of the compound or the salt; and an antibacterial drug and an infectious disease treating drug containing the compound as an active ingredient.