Disclosed in the present specification is a method capable of preparing N-[4-[(1R)-1-[[(R)-(1, 1-dimethylethyl)sulfinyl]amino]ethyl]-2, 6-difluorophenyl]-methanesulfonamide (INT028-2) with high optical purity, through the selection of Ellman-chiral auxiliaries and the re-crystallization and separation of optical isomers. According to the above method, high-purity N-[4-[(1R)-1-[[(R)-(1, 1-dimethylethyl)sulfinyl]amino]ethyl]-2, 6-difluorophenyl]-methanesulfonamide with excellent quality can be produced at room temperature by improving cryogenic process conditions necessary for realizing high optical purity, and thus the trimming due to the process failure rate can be remarkably reduced.