To obtain an intermediate for synthesizing cephalosporin useful as e.g. an antibacterial agent, in a site-specific way and in high yield, by reaction of a specific 3-halomethyl-3-cephem-4-carboxylic acid compound with a specifically reactive formamide derivative.

A 7-β-protected amino-3-halomethyl-3-cephem-4-carboxylic acid ester compound (salt) of formula I (R1 is an amino-protecting group; R2 is a carboxylic acid-protecting group; X is a halogen) is reacted with a reactive formamide derivative bearing a group of formula II [R3 and R4 are each a (substituted) hydrocarbon or may form a ring together with the adjacent nitrogen atom] in the presence of a metal (salt) having reducing power such as a stannous halide or metallic tin, zinc, manganese or iron to obtain the objective intermediate of formula III for synthesizing cephalosporin-based compounds useful as antibacterial agents in a site-specific way and in high yield.