This invention provides a process for preparing 3-exomethylenecepham derivatives represented by the formula ##STR1## wherein R.sup.1 is arylacetylamino, aryloxyacetylamino, heterocyclic acetylamino or imido, and R.sup.2 is a protective group for the carboxyl, the process being characterized in that a 3-halomethylcephem compound represented by the formula ##STR2## wherein R.sup.1 and R.sup.2 are as defined above, and X is a halogen atom is electrolyzed in a mixture of a hydrophilic organic solvent and water.