A process for producing a compound of the formula (I) ##SPC1## wherein R.sub.2 is a hydrogen atom or an ester protective group, which is useful as a precursor for the production of cephalosporin derivatives, comprising reacting a phosphoramide derivative of cephalosporin of the formula (II) ##SPC2## wherein R.sub.1 is a lower alkyl group and R.sub.2 is as defined above, with a phosphorus acid. A further embodiment includes preparing the phosphoramide derivative of cephalosporin of the formula (II) by reactng a compound of the formula (III) ##SPC3## wherein R.sub.1 and R.sub.2 are as defined above, with an acid compound.