PURPOSE: To industrially and advantageously obtain the title compound useful as a synthetic raw material for medicines and agricultural chemicals by treating a mixture of a mandelonitrile or a benzaldehyde of racemic modification and prussic acid with a bacterium capable of hydrating a nitrile group.

CONSTITUTION: A mixture of a mandelonitrile of racemic modification shown by formula I (X is H, methyl, methoxy, hydroxyl or halogen) or a derivative thereof (e.g. 2-chloromandelonitrile) or a benzaldehyde shown by formula II or a derivative thereof (e.g. 2-chlorobenzaldehyde) of racemic modification and prussic acid is treated in a neutral or basic poloar solvent with a bacterium [e.g. Rhodococcus sp. PN 42-2 (FERM P-11,775)] capable of stereospecifically hydrating nitrile group of the mandelonitrile of racemic modification shown by formula I or the derivative thereof or a treated material thereof to directly give an advantageously amount of optically active S-(+)-maldelamide shown by formula III or a derivative thereof.