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.Se proporciona un derivado novedoso de ácido piridonacarboxílico o una sal del mismo que tiene un excelente efecto sobre bacterias como el Staphylococcus aureus resistente a la meticilina, Pseudomonas aeruginosa y Clostridium difficile y no es susceptible a las bombas de flujo de fármacos y un agente antimicrobiano que lo comprende； un derivado de ácido piridonacarboxílico representado por la fórmula (1), en donde R1 es un átomo de hidrógeno, un átomo de halógeno, un grupo alquilo inferior o un grupo amino； R2 es un grupo -NH-R6, en donde R6 es un átomo de hidrógeno, un grupo alquilo inferior, un grupo alquilamino inferi
