To provide an efficient and industrial method for the separation of an N-substituted homophenylalanine's optical isomer mixture.

The method comprises contacting an optical isomer mixture of N-benzyloxycarbonyl homophenylalanine with a preparative optical isomer separating agent comprising a compound selected from the group consisting of optically active polymaleimide derivatives substituted with optically active phenylethyl group, cyclohexylmethyl group and benzyloxycyclopentyl group.