To efficiently and conveniently separate a high purity sulfoxide- containing diasteremer purified with specific tetrakis phenols.

A diastereomer is separated in a purified state with tetrakis phenols of formula I [wherein, X is (CH2)n or phenylene; (n) is 0-3; R1 and R2 are each H, a lower alkyl or the like]. The diastereomer to be separated is preferably separated as a molecular compound with the tetrakis phenols of formula I, is a compound containing a sulfoxide group and is preferably a compound of formula II [wherein, Y is (CH2)n or phenylene; (n) is 0-8; X and Z are each a 1-8C alkyl or the like]. The compound of formula I is e.g. 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane or the like. The separation procedure is carried out by stirring a mixture consisting of two or more kinds of diastereomers in the presence of a reactive solvent (e.g. water, methanol or the like) at a temperature from normal temperature to 100°C for a few minutes to a few hours to cause reaction.