Provided is a method for separating the four optical isomers of Nahlsgen. The method according to the present invention for producing a mixture of a D-2-amino-4-[(Rp)-(3-carboxymethylphenoxy)(methoxy)phosphoryl]butanoic acid hydrate and an L-2-amino-4-[(Sp)-(3-carboxymethylphenoxy)(methoxy)phosphoryl]butanoic acid hydrate includes subjecting a mixture of the four optical isomers, represented by Formula (1'), to fractional crystallization from water or from a solvent mixture of water and a water-soluble organic solvent, to precipitate a mixture of a compound represented by Formula (1-1'-1) and a compound represented by Formula (1-4'-1), where Formulae (1'), (1-1'-1), and (1-4'-1) are expressed as follows:
