Abstract: The present invention provides a simple industrial process for producing an L- or D-optically active ?-methylcysteine derivative or its salt, which is a useful pharmaceutical intermediate, from readily available, inexpensive raw materials. In a process for producing an L- or D-optically active ?-methylcysteine derivative or its salt, a racemic N-carbamoyl-?-methylcysteine derivative or its salt is D-selectively cyclized with hydantoinase to produce a D-5-methyl-5-thiomethylhydantoin derivative or its salt and an N-carbamoyl-?-methyl-L-cysteine derivative or its salt, which are then subjected to deprotection of the amino group and the sulfur atom, and hydrolysis.