The present invention provides with a process of preparing an optically active succinimide derivative, which is a key intermediate for production of ranirestat. A compound (3) is easily prepared by treating the derivative of succinic acid diester of the formula (2): wherein R1 is an amino group protected with a group removed by hydrogenolysis or a tert-butoxycarbonylamino group and R2 is an ethyl group optionally substituted with one or two methyl group(s) at α-position, provided that R2 is not a tert-butyl group when R1 is a tert-butoxycarbonylamino group; with alkali metal alkoxide and the compound (3) can be an important intermediate for production of ranirestat.