An industrially excellent, novel process for producing cephem compounds which are useful as medicines, particularly, antibiotics and represented by the following formula (III): ##STR1## wherein R.sup.1 means either group represented by the following formula: ##STR2## R.sup.2 denotes a carboxyl group or a carboxyl anion, and R.sup.3 stands for a lower alkyl group, a halogen atom, a lower alkyl group substituted by an aliphatic acyloxy group having 1-6 carbon atoms, or any one of the groups represented by the following formulae: ##STR3## which comprises reacting in water a 3-cephem-4-carboxylate represented by the following formula (I): ##STR4## or a salt thereof with an acid chloride represented by the formula (II), R.sup.1 COCl.