An objective of the present invention is to provide compounds of formula (I) or their pharmacologically acceptable salts or solvates wherein A represents aryl while R 1 represents a five- or six-membered monocyclic heterocyclic group, or A represents a four- to six-membered monocyclic heterocyclic group while R 1 represents aryl or a five- or six-membered monocyclic heterocyclic group； R 2 represents a hydrogen atom or C 1-6 alkyl； R 3 represents C 1-6 alkyl or C 3-6 cycloalkyl-C 1-4 alkyl； R 4 , R 5 , and R 6 represent a hydrogen atom； R 7 represents C 1-6 alkyl； and m is 1 to 3. The compounds are novel lincosamide derivatives that have a potent activity against resistant Streptococcus pneumoniae. Further, the compounds are usable as antimicrobial agents and are useful for preventing or treating bacterial infectious diseases.
