Provided are a novel pyridonecarboxylic acid derivative which is highly effective even against bacteria such as methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa, and Clostridium difficile, and is less likely to be affected by a drug efflux pump, or a salt thereof, and an antibacterial agent containing the pyridonecarboxylic acid derivative or a salt thereof. The pyridonecarboxylic acid derivative is represented by formula (1): (in the formula, R1 represents a hydrogen atom, a halogen atom, a lower alkyl group, or an amino group； R2 represents radical-NH-R6 (here, R6 represents a hydrogen atom, a lower alkyl group, an amino lower alkyl group, etc.), radical-O-R7 (here, R7 represents a hydrogen atom, a lower alkyl group, etc.), radical-(CH2)m-R8 (here, R8 represents an amino group, etc., m represents an integer of 1-4), or a cyclic amino group represented by formula (2): (in the formula, Y represents NH or C-R9aR9b (here, R9a and R9b each independently represent a hydrogen atom, a lower alkyl group, an amino group, a lower alkylamino group, etc.), and n and p are an integer of 1 or 2)； R3 represents a hydrogen atom, a halogen atom, a lower alkyl group, etc.； R4 represents a hydrogen atom or a carboxyl group-protecting group； R5 represents a hydrogen atom or a hydroxy group-protecting group). Alternatively, a salt thereof is provided.L'invention concerne un nouveau dérivé de l'acide pyridonecarboxylique qui est hautement plus efficace même contre les bactéries tel que les méticillino-résistantes Staphylococcus aureus , Pseudomonas aeruginosa ,et Clostridium difficile et est moins susceptible d'être affecté par un médicament à efflux pompe, ou son sel, et un agent antibactérien contenant un dérivé de l'acide pyridonecarboxylique ou son sel correspondant. Le dérivé d'acide pyridonecarboxylique est représenté par la formule (1) : (dans la formule, R 1 représente un atome d'hydrogène, un atome d'halogène, un groupe alkyle inférieur ou un groupe a
