PURPOSE: To obtain the titled compound which is a kind of protected amino acid useful for peptide synthesis, suppressing racemization in high yield, by adding a specific reaction reagent to the reaction system, thereby suppressing the hydrolysis of benzyl halide even in a system containing water.

CONSTITUTION: An aqueous solution of an alkali metal hydroxide is added to a reaction system containing D- or L-N-t-butoxycarbonylserine and a benzyl halide or, as an alternative method, a benzyl halide and an aqueous solution of an alkali metal hydroxide are simultaneously added to a reaction system containing D- or L-N-t-butoxycarbonylserine and the components are made to react with each other in a solvent such as dimethylformamide at 0W30°C for 0.5W3hr to obtain the objective compound. Since alkali metal hydroxide has high solubility in aqueous solvent, the reaction can be carried out without using a large amount of solvent and the recovery of the solvent is facilitated.