A compound of the formula: ##STR1## wherein R.sup.2 is hydrogen or a halogen and R.sup.3 stands for methyl, acetoxymethyl, carbamoyloxymethyl, an alkoxymethyl, an alkylthiomethyl, 2-carboxy-1-ethenyl, or a heterocyclic thiomethyl, or a pharmaceutically acceptable salt thereof, is found to have a broad antimicrobial spectrum and, in particular, effective against Gram-negative bacteria including Escherichia coli, Proteus vulgaris, Pseudomonas aeruginosa, Serratia marcescense as well as Gram-positive ones including antibiotic resistant strains. Thus, these compounds may be used for antimicrobial agents in therapeutical purposes.