The present invention provides a simple and inexpensive method for producing .alpha.-hydroxy-.beta.-aminocarboxylic acids and their esters.An ester of an N-protected .alpha.-amino acid ester is converted into a .beta.-ketosulfoxide, which is then processed with an acid to give an .alpha.-ketohemimercaptal. Next, this is acylated and then processed with a base to obtain an N-protected .alpha.-acyloxy-.beta.-amino-thioester, which is then saponified to obtain an intended compound. According to the method of the present invention, it is possible to produce .alpha.-hydroxy-.beta.-aminocarboxylic acid derivatives, which are intermediates in producing various HIV protease inhibitors, renin inhibitors and carcinostatics, from a-amino acids. The method comprises reduced reaction steps, the selectivity in the method to give the intended product is high, and the yield of the product obtained is high.