An optically active pyridobenzoxazine derivative represented by formula (I): ##STR1## wherein R.sup.1 and R.sup.2 wherein R.sup.1 and R.sup.2, which may be the same or different, each represent an alkyl group having from 1 to 6 carbon atoms, a halogenated alkyl group having from 1 to 6 carbon atoms, a hydroxyalkyl group having from 1 to 6 carbon atoms, or an alkoxymethyl group having from 2 to 7 carbon atoms; R.sup.3 represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, an acyl group having from 2 to 7 carbon atoms, a halogen-substituted or unsubstituted alkyloxycarbonyl group having from 2 to 7 carbon atoms, a phenylmethyloxycarbonyl group, or a phenylmethyloxycarbonyl group whose phenyl moiety is substituted with a halogen atom, a nitro group, or a methoxy group; R.sup.4 represents a methyl or fluoromethyl group which is in a .beta.-configuration; R.sup.5 represents a hydrogen atom, an amino group, a hydroxyl group, a methyl group, or a halogen atom; and X represents a fluorine atom or a chlorine atom, and a salt thereof are disclosed. These compounds exhibit potent antimicrobial activity with broad antimicrobial spectra and improved lipophilic properties.