6-Hydroxy nitrogen-containing 6-membered ring compounds of the following general formula (II): ##STR1## wherein R.sup.1 represents carboxy group, carbamoyl group, cyano group, formyl group, C.sub.1 -C.sub.5 hydroxyalkyl group, C.sub.2 -C.sub.6 alkoxycarbonyl group, carboxyvinyl group, carboxymethyl group or oxime group, R.sup.2 represents hydrogen atom or carboxy group, and A represents carbon atom or nitrogen atom, can be prepared by reacting a nitrogen-containing 6-membered ring compounds of the following general formula (I): ##STR2## wherein R.sup.1, R.sup.2 and A are as defined in the general formula (II) above, with a microorganism or physico-chemically treated microorganism in an aqueous medium. Efficiency of the above reaction can be raised by conducting the reaction in the presence of phenazine methosulfate.