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: