PURPOSE: To simply obtain an aziridine compound, especially an optically active substance at a low cost by using a sulfonamide compound having thioether group and sulfonamide group bonded to the adjacent carbons, especially an optically active substance and forming the aziridine ring.

CONSTITUTION: A sulfonamide compound having thioether group and sulfonamide group bonded to the adjacent carbons, especially preferably an optically active substance, e.g. N-[1-phenyl-2-phenylthio)ethyl]-p-toluenesulfonamide is reacted with a methylating agent (especially trimethyloxonium tetrafluoroborate to provide a sulfonium salt, which is then reacted with a basic compound (especially sodium hydride) to form the aziridine ring. Thereby, an aziridine compound [e.g. 1-(p-tolylsulfonyl)-2-phenyl-aziridine] is obtained. The optically active aziridine compound is useful for producing physiologically active substances, functional compounds, etc., at a low cost.