A process for the preparation of a 3-vinylcephem compound, characterized by treating a protected 3-vinylcephem derivative of the general formula (1) in an organic solvent in the presence of a perhalogenic acid and an organic protonic acid to thereby obtain a 3-vinylcephem compound of formula (2). In general formula (1), R?1, R2 and R3¿ are each hydrogen or optionally substituted arylmethyl, with the proviso that all of R?1, R2 and R3¿ must not be hydrogen.