A process for preparing an optically active mercapto compound having asymmetrical centers having the formula: ##STR1## in which R.sub.1 represents an alkyl group, R represents a hydrogen atom or an alkyl group, C* represents an asymmetric carbon, n represents the value of 1 or 2, is disclosed. The process involves the reaction of a cultured microorganism having the ability of asymmetrically hydrolyzing ester bonds of the raw material, thus eliminating the need for protecting free mercapto groups which are unstable in the reaction conditions. The optically active mercapto compound is useful as a raw material for producing a compound having medicinal effects such as antihypertensive activity.