The present invention relates to a process for producing a xanthine derivative represented by formula (II), comprising converting a xanthine derivative represented by formula (I) {hereinafter, referred to as Compound (I)}: ##STR1## (wherein R.sup.1 and R.sup.2 independently represent hydrogen, or hydroxy-substituted, oxo-substituted, or unsubstituted lower alkyl) into a xanthine derivative represented by formula (II) {hereinafter, referred to as Compound (II)}: ##STR2## (wherein R.sup.3 and R.sup.4 independently represent hydrogen, or hydroxy-substituted, oxo-substituted, or unsubstituted lower alkyl; R.sup.5 and R.sup.6 independently represent hydrogen, hydroxy, or oxo; with the proviso that R.sup.5 and R.sup.6 are both hydrogen, at least one of R.sup.3 and R.sup.4 is hydroxy-substituted or oxo-substituted lower alkyl; and X and Y both represent hydrogen or are combined with each other to form a single bond) in the presence of an enzyme source for catalyzing hydroxylation or carbonylation of Compound (I) into Compound (II), and collecting the produced Compound (II).