PURPOSE: To obtain the titled compound useful as a food additive, etc. advantageously in a small-sized reactor without requiring concentrating steps, by reacting glycine with acetaldehyde in a slurry state in the presence of a copper salt and a base in an aqueous medium.

CONSTITUTION: Glycine is reacted with acetaldehyde in the presence of a copper salt and a base or copper glycinate is reacted with the acetaldehyde in the presence of a base to give an α-amino-β-hydroxybutyric acid (hereinafter abbreviated to theronine). In the process, the reaction is carried out using the glycine or copper glycinate in a slurry state in an aqueous medium. The above-mentioned reaction is advanced enough smoothly by using water which is a reaction solvent in an amount to make the glycine or copper glycinate in the slurry state, i.e. preferably within 2W7 weight ratio based on the glycine, and the reactor having (1/2):(1/4) volume of that of the conventional reactor can be used without requiring an apparatus for concentrating steps.