The present invention relates to a process for producing optically active diols represented by the general formula (II): ##STR1## wherein R.sup.1 represents (CH.sub.2).sub.n, CH.dbd.CH, O, S or NH whereupon n is an integer of 1 to 4, and R.sup.2 represents hydrogen, C.sub.1-6 alkyl, C.sub.1-6 alkoxy, (C.sub.1-6 alkoxy)-carbonyl, hydroxy, carboxy, halogen, nitro or amino, which comprises treating compounds represented by the general formula (I): ##STR2## wherein R.sup.1 and R.sup.2 have the same meanings as defined above, with a culture of a microorganism belonging to the genus Rhodococcus, Bacillus, Brevibacterium or Gordona and being capable of stereoselectively diolating a double bond in ring A, or with a culture of Mortierella vinacea, or with said microorganism itself, or with a treated material from said microorganism.