PURPOSE: To industrially obtain a drug having high optical purity by treating a specific compound with a microorganism belonging to the genus Ashbya, Brettanomyces or Botryoascus.

CONSTITUTION: One or more strains selected from Ashbya gossypii, Brettano myces custersianus, Botryoascus synnaedendrus, Candida curvata, Candida krusei, Cryptococcus albidus, Clavispora lusitaniae or Rhodococcus erythropolis is cultured to give a cell (A). Then the component A is reacted with a substrate (B) containing 0.1-10w/v% benzoylacetic acid derivative shown by formula I (R is straight-chain or branched chain alkyl; X is H, halogen, nitro, etc.) at 15-50°C at pH4-9 for 2-96 hours while aerating and stirring to give a reaction product (C). Then the component C is extracted and purified to produce an optically active (S)-3-phenyl-3-hydroxypropionic acid derivative shown by formula II.