PURPOSE: To prepare L-mandelic acid, etc. having high optical purity, in high resolution efficiency, by adding L-cysteine, etc. to a solution containing DL- cysteine and DL-mandelic acid, crystallizing the L-cysteine.L-mandelic acid complex, etc., and separating the reaction mixture into solid and liquid.

CONSTITUTION: DL-cysteine and DL-mandelic acid are dissolved in water, methanol, etc. L-cysteine.L-mandelic acid complex or D-cysteine.D-mandelic acid complex is crystallized from the solution by adding one or more substances selected from L-cysteine, D-cysteine, L-mandelic acid, and D-mandelic acid to the solution, or by adding a cysteine salt other than the cysteine.mandelic acid complex to the solution as a co-existing salt. The crystallized solution is separated into solid and liquid, and L- or D-cysteine and L- or D-mandelic acid are separated therefrom.

USE: Raw material of pharmaceuticals, optical resolution agent, etc.