A process for producing a labeled precursor which is useful for production of a radioactive fluorine-labeled amino acid compound is provided. In the reaction step for introducing a leaving group to a mixture of syn-form and anti-form of FACBC, a base is allowed to present in the reaction system to produce a syn-leaving group adduct which is unreactive with the base and is highly stable and an anti-leaving group adduct which can react with the base to form a water-soluble compound. By employing a purification method utilizing such a difference in solubility, the syn-leaving group adduct can be separated selectively. The base may be a linear-chain or branched-chain primary to tertiary alkylamine having 1 to 10 carbon atoms, a nitrogen-containing heterocyclic compound with 2 to 20 carbon atoms, and a nitrogen-containing hetero aromatic compound with 2 to 20 carbon atoms.
