A method for producing a compound of the formula; ##STR1## wherein R stands for a hydrogen atom, an acyl group or a protective group other than acyl groups, Q stands for a hydrogen atom or an ester residue, Y stands for the residue of a nucleophilic compound and the dotted line shows the double bond at 2- or 3- position of the cephem ring or a salt thereof, characterized by allowing a compound of the formula; ##STR2## [R, Q and the dotted line are of the same meaning as above] or a salt thereof to react with a nucleophilic compound or a salt thereof and a compound of the formula; ##STR3## wherein R.sup.1, R.sup.2, R.sup.3 independently stand for a hydrocarbon group having not more than 8 carbon atoms, or R.sup.1 and R.sup.2, R.sup.1 and R.sup.3 or R.sup.2 and R.sup.3 may be combined to form polymethylene group, according to which the end product [I] of high quality can be provided in a high yield by one reaction step by using the compound [III] of relatively low cost, thus the method of this invention is advantageous for mass-producing the compound [I] on an industrial scale from the compound [II] and a nucleophilic compound.