There is provided a process for producing an optically active 2-alkylcysteine or a salt thereof, characterized by allowing cells of microorganism or treated products thereof having an activity of stereoselective hydrolysis of the amide bond of a 2-alkyl-L-cysteinamide or a salt thereof to act on a 2-alkylcysteinamide consisting of a mixture of D- and L-isomers or a salt thereof; and allowing the obtained 2-alkyl-L-cysteine and 2-alkyl-D-cysteinamide to react with an aldehyde or a ketone, or an acetal or ketal thereof, so as to derive therefrom a 4-alkylthiazolidine-4-carboxylic acid or a salt thereof and a 4-alkylthiazolidine-4-carboxamide or a salt thereof, thereby efficiently separating and obtaining a 2-alkyl-L-cysteine or a salt thereof, or a 2-alkyl-D-cysteine or a salt thereof. There is also provided a process for producing a 4-alkylthiazolidine-4-carboxylic acid or a salt thereof from the 2-alkylcysteine or a salt thereof.