New intermediate, an optically active (1S,5R)-7,7-alkylenedioxy-2-alkoxycarbonylbicyclo[3.3.0]octan-3-one which is useful for the synthesis of an optically active carbacyclin. This intermediate is prepared, as a non-reduced compound, from a racemic compound, (1SR,5RS)-7,7-alkylenedioxy-2-alkoxycarbonyl-cis-bicyclo[3.3.0]octane-3-one, by treatment of the racemic compound with a microorganism. The microorganism has an ability of specifically reducing the keto group of the (1R,5S) epimer of the racemic compound.