Novel 1-substituted phenyl-4-oxoquinoline-3-carboxylic acid compounds of the formula: ##STR1## wherein R.sup.1 is hydrogen atom or an alkyl having 1 to 2 carbon atoms, R.sup.2 is hydrogen atom or fluorine atom, and R.sup.3 is hydroxy, fluorine atom or an alkanoyloxy having 1 to 6 carbon atoms, and a pharmaceutically acceptable salt thereof, said compounds having excellent antimicrobial activity and hence being useful as an antimicrobial agent, and a pharmaceutical composition comprising as an active ingredient said 1-substituted phenyl-4-oxoquinoline-3-carboxylic acid compounds or a pharmaceutically acceptable salt thereof in admixture with a pharmaceutically acceptable diluent or carrier.