Abstract: The present invention provides a process for preparing cephem derivatives having the following general formula (I): ##STR1## in which R.sup.1 represents a carboxy group or a protected carboxy group which can form the salt of --COO.sup.- M.sup.+ with an alkali metal ion (M.sup.+) such as sodium, or may represent --COO.sup.- when R.sup.2 has a substituent having positive electric charge such as pyridinium, pyrimidinium or thiazolium,R.sup.2 represents hydrogen, acyloxymethyl, heterocyclic methyl or heterocyclic thiomethyl, each of which can be substituted with appropriate substituents,R.sup.3 represents hydrogen or an amino-protecting group,R.sup.4 represents C.sub.1 -C.sub.4 alkyl or phenyl, or together with the oxygen or phosphorus atom to which it is attached may form a 5- or 6-membered heterocyclic ring, andQ represents N or CH, characterized in that a reactive thiophosphate derivative of thia(dia)zole acetic acid having the following general formula (II): ##STR2## wherein R.sup.3, R.sup.