A process for producing an optically active 2-hydroxybutyric ester (1), characterized by including reacting an optically active 2,3-epoxybutyric ester (2) with a thiol in the presence of scandium trifluoromethanesulfonate or ytterbium trifluoromethanesulfonate, to thereby produce Compound (3), and subjecting Compound (3) to desulfurization reaction: ##STR00001## (wherein R represents a C1 to C6 alkyl group or a C7 or C8 aralkyl group; R.sup.1 represents a C1 to C12 alkyl group or a phenyl group; and * represents S- or R-absolute configuration). The present invention provides a process for producing an optically active 2-hydroxybutyric ester at high yield and high optical purity.