An improved process for preparing an optically active 4-hydroxycyclopentenone of the formula: ##STR1## wherein R is a lower alkyl, the symbol means double bond or triple bond, and the * marked carbon is an asymmetric carbon, and the corresponding racemic 4-hydroxycyclopentenone, which are useful as an intermediate for preparing medicinal or agricultural products, particularly pharmaceutically active prostaglandins, and intermediates for preparing the optically active and/or racemic 4-hydroxycyclopentenone.