The present invention relates to a new process for preparing beta-ketoester compound of the following formula (1), which is a useful intermediate for the synthesis of such quinoline antibiotics as Ciprofloxacin, Levofloxacin, Gemifloxacin, Trovafloxacin, etc. The quinoline antibiotics obtained from the above compound of formula (1) show potent antibacterial activity, and so are advantageously used as a therapeutic agent for bacterial infections of humans or animals.