The present invention provides a method for preparing optically active 3-hydroxypentanenitrile with high yield. Optically active 3-hydroxypentanenitrile is prepared by stereoselectively reducing 3-ketopentanenitrile by action of an enzyme, which asymmetrically reduces 3-ketopentanenitrile to optically active 3-hydroxypentanenitrile. Also, alkali metal salt of 3-ketopentanenitrile, which is a stable compound without problems regarding storage, can be efficiently obtained.