The present invention relates to novel cephalosporin derivatives, processes for preparing thereof, compositions for preventing and/or treating infectious diseases which comprise the novel cephalosporin derivatives as active components, and the intermediate compounds in the synthesis of cephalosporin derivatives and processes for producing thereof.The novel cephalosporin derivatives according to the present invention contain condensed heterocyclic groups, particularly a triazolopyrimidine ring or a thiadiazolopyrimidine ring as substituents at the 3-position of the cephem skeleton, and a hydroxyimino, an alkyloxyimino or an acyloxyimino moiety as substituents at the 7-position of the cephem skeleton.The compounds of the present invention containing the aforementioned substituents have a strong antibacterial activity against gram-negative bacteria and also against gram-positive bacteria including methicillin-resistant Staphylococcus aureus. These compounds are extremely useful for the treatment of infectious diseases.