PURPOSE: To enable obtaining of a high optical purity and rate of reaction at a low cost by reacting a racemic 2-hydroxynitrile with a microorganism of the genus Bacillus, etc., at the prescribed rate of reaction.

CONSTITUTION: A racemic 2-hydroxynitrile (A) expressed by formula I [R is (un)substituted aryl or (un)substituted heterocyclic group] in an amount of 0.01-70wt.% is added to an aqueous medium to provide a raw material solution (B). On the other hand, a microorganism of the genus Alcaligenes, Acinetobacter or Bacillus is cultured in a culture medium to afford cultured microbial cells (C). The resultant microbial cells (C) in an amount of 0.05-20wt.% are added to the solution (B) and reacted at pH4-11 and 5-80°C for 1-100hr until the rate of reaction attains ≥50% to 100%. Thereby, a reaction completed solution (D) is obtained. The prepared ingredient (D) is then filtered to remove undissolved substances such as micrcbial cells. Impurities are subsequently extracted and removed with benzene, etc., at pH about 8.5 and the obtained solution is extracted with chloroform, etc., at pH about 2 to afford an extract (E), which is then purified by column chromatography to produce the objective optically active 2-hydroxycarboxylic acid expressed by formula II.