To obtain an N-tertiary butoxycarbonylphenylalanine ester having high optical purity.

This method for producing an N-tertiary butoxycarbonylphenylanine ester having high optical purity comprises (1) a process for reacting a phenylalanine ester salt with phosgene in an organic solvent and introducing carbonyl group into an amino group of the phenylalanine ester to produce an N-carbonylphenylalanine ester, (2) a process for reacting N-carbonylphenylalanine ester obtained in the process (1) with tert-butanol to produce a crude N-tert-butoxycarbonylphenylalanine ester and (3) a process for extracting crude N-tert-butoxycarbonylphenylalanine ester obtained in the process (2) with an aliphatic hydrocarbon to purity the N-tert- butoxycarbonylphenylalanine ester.