The present invention provides a simple industrial process for producing an L- or D-optionally active alpha -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 alpha - methylcysteine derivative or its salt, a racemic N-carbamoyl- alpha - 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- alpha -methyl-L-cysteine derivative or its salt, which are then subjected to deprotection of the amino group and the sulfur atom, and hydrolysis.