An isolable activated ester, diethyl thiophosphoryl [(Z)-(2-aminothiazol-4-yl)-2-(methoxyimino)acetate (9a, DAMA) was prepared in high purity, which was at least stable for 7 d at $25^{circ}C$ and 6 months at $5^{circ}C$. Its reaction with 7-ACA (4) in the presence of tributylamine in i-PrOH provided cefotaxime (1) of high purity in good yield. Preparation of DAMA and its reaction with 7-ACA (4) to 1 was performed in one-pot manner, respectively with minimal unit operations of stirring and filtration.