A process for preparing a 3-halogenated cephem derivative represented by the formula (2), characterized by causing a halogenating reagent to act on an allenyl .beta.-lactam compound represented by the formula (1) in the presence of a cuprous salt or cupric salt to obtain the 3-halogenated cephem derivative ##STR1## wherein R.sup.1 is a hydrogen atom, amino or protected amino, R.sup.2 is a hydrogen atom, halogen atom, lower alkoxyl, lower acyl, lower alkyl, lower alkyl having at least one selected from hydroxyl and protected hydroxyl as a substituent, hydroxyl or protected hydroxyl, R.sup.3 is a hydrogen atom or carboxylic acid protecting group, R.sup.4 is an aromatic compound residue which may have a substituent or nitrogen-containing aromatic heterocyclic compound residue which may have a substituent, and n is 0 to 2 ##STR2## wherein R.sup.1, R.sup.2 and R.sup.3 are as defined above, X is a halogen atom.