Method of producing proton pump inhibitor compound having optical activity

A highly pure optically active proton pump inhibitor compound can be produced safely and inexpensively in a high yield and enantioselectivity by a method of producing an optically active sulfoxide of Formula 2 or a salt thereof, comprising oxidizing a sulfide of Formula 1 or a salt thereof with hydrogen peroxide using an iron salt in the presence of a chiral ligand of Formula 3; wherein A is CH or N; R 1 is hydrogen atom, an alkyl optionally substituted by halogen(s), or an alkoxy optionally substituted by halogen(s); one to three R 2 may exist, and each of R 2 is independently an alkyl, a dialkylamino, or an alkoxy optionally substituted by halogen(s) or alkoxy(s); each of R 3 is independently hydrogen atom, a halogen, cyano or the like; R 4 is a tertiary alkyl; and * and ** represent respectively R configuration or S configuration.