Résumé dans la langue de publication Cephem derivatives or pharmaceutically acceptable salts thereof represented by the formula I: wherein represents benzene ring, pyridine ring, pyrazine ring or 5-membered aromatic heterocycle (having one oxygen or sulfur atom as ring-constituting atom); X and Y respectively represent hydrogen atoms or CXY represents C=N-OH; R1 represents phenyl, thienyl or thiazolyl (which may be substituted with amino group or halogen atom); and R2 , R3 and R4 respectively represent hydrogen atoms, halogen, hydroxy C1 -C6 alkyl, isothiuronium C1 -C6 alkyl, amino C1 -C6 alkyl or amino C1 -C6 alkylthiomethyl, there being no R4 where represents 5-membered aromatic heterocycle.