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 R1 represents a five- or six-membered monocyclic heterocyclic group, or A represents a four- to six-membered monocyclic heterocyclic group while R1 represents aryl or a five- or six-membered monocyclic heterocyclic group; R2 represents a hydrogen atom or C1-6 alkyl; R3 represents C1-6 alkyl or C3-6 cycloalkyl-C1-4 alkyl; R4, R5, and R6 represent a hydrogen atom; R7 represents C1-6 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.