A 3-�(E)-1-propenyl!cephem compound represented by formula (I): ##STR1## wherein R represents hydrogen, a protective group for an amino group, or the group shown by formula (II): ##STR2## in which R.sup.3 is a protective group for an amino group, R.sup.4 is a protective group for a hydroxyl group, and W is --CH.dbd. or --N.dbd.; R.sup.2 represents a protective group for a carboxyl group; and X represents hydrogen or chlorine. The compound is useful as an intermediate for producing cephem antibiotics and is prepared by isomerizing the corresponding 3-�(Z)-1-propenyl!cephem compound in an inert organic solvent in the presence of an aromatic thiol. The compound of formula (I) wherein X is chlorine can be converted into a 3-�(E)-3-ammonio-1-propenyl!cephem derivative through the reaction with a tertiary amine.