Pyridonecarboxylic acid derivatives or salts thereof

The present invention provides an agent which exhibits excellent antibacterial activity, low toxicity, improved bioavailability, and low binding rate to serum proteins. The present invention is directed to a pyridonecarboxylic acid derivative represented by formula (1): or a salt thereof, wherein R1 represents a methyl group, a fluorine atom, or a chlorine atom; R2 represents a hydrogen atom or a lower alkyl group; R3 represents an isopropyl group or a tert-butyl group; R4 represents a methyl group or a halogen atom; and R5 represents a fluorine atom or a chlorine atom. The present invention is also directed to an antibacterial agent and a medicament containing the derivative or the salt thereof as an active ingredient.