This invention provides a process for preparing 2-oxycephalosporin derivative represented by the formula ##STR1## wherein R.sup.1 represents hydrogen atom, acyl group, or lower alkoxycarbonyl group optionally substituted with halogen atom, R.sup.2 represents hydrogen atom, halogen atom or acyloxy group optionally substituted with halogen atom, R.sup.3 represents hydrogen atom, lower alkyl group optionally substituted with halogen atom, or phenyl-lower alkyl group which may be optionally substituted with nitro group, halogen atom or lower alkoxy group on the phenyl ring, and R.sup.4 represents lower primary or secondary alkyl or lower alkylcarbonyl, the process comprising electrolytically oxidizing a cephalosporin derivative represented by the formula ##STR2## wherein R.sup.1, R.sup.2 and R.sup.3 are as defined above, in the presence of a lower carboxylic acid or lower primary or secondary alcohol, and a supporting electrolyte.