The present invention relates to a process for preparing a beta-keto ester compound of formula (1), which is an intermediate for the synthesis of quinolone antibiotics. Particularly, the present invention is characterized by the reaction of an organo nitrile compound with a salt of mono-alkyl malonate in the presence of metal salt, and so the reaction is easy to control due to its endothermic nature, and is devoid of lachrymatory reagents with excellent reproducibility. Subsequent in situ hydrolysis in the presence of aqueous acid solution provided the compound of formula (1).