An optically active amino acid is useful as food or feed, agrochemicals, chemical products for industrial use, intermediates for synthesis of cosmetics or medicines and the like and is also important as optical resolving agents or chiral building blocks for use in organic synthesis. Thus, the object is to provide an industrially practical method for producing the optically active amino acid simply and at low cost. The method comprises the step of reacting an aminonitrile composed of a mixture of a D-aminonitrile and an L-aminonitrile with a biocatalyst which is one derived from a newly isolated microorganism belonging to the genus Rhodococcus and has an activity of converting the two aminonitriles into a D-amino acid amide and an L-amino acid amide respectively, a biocatalyst which has an activity of racemizing the D-amino acid amide and the L-amino acid amide to each other, and a biocatalyst which has an activity of converting one of the D-amino acid amide and the L-amino acid amide into the corresponding D- or L-amino acid.