To provide a method for producing an amidothiazole derivative ester compound, by which the amidothiazole derivative ester compound useful as an intermediate for cephalosporin-based antibiotics can simply be obtained in a high purity.

This method for producing the amidothiazole derivative ester compound such as ethyl 2-(2-tert-butoxycarbonyl-L-alanylaminothiazol-4-yl)-2-(Z)- methoxyiminoacetate, comprising condensing an N-protected amino acid compound such as tert-butoxycarbonyl-L-alanine with an aminothiazole derivative ester compound such as ethyl 2-(2-aminothiazol-4-yl)-2-(Z)-methoxyiminoacetate in the presence of a condensing agent, is characterized by using a combination of a dicarbonate compound such as di-tert-butyldicarboxylate with a tert-amine compound such as N-methymorpholine as the condensing agent.