PURPOSE: To obtain easily and economically the titled compound useful as a coronary vasodilator, antidepressant, tranquilizer, etc., by reacting a racemic modification of a benzothiazepine derivative with a chiral large-ring polyether, and separating the reaction product into d- and l-isomers.

CONSTITUTION: A racemic modification of a benzothiazepine derivative expressed by formula I [R1 is H or COCH3; R2 is H or CH2CH2N(CH3)2] is combined with the S- or R-form of a chiral large-ring polyether (a crown compound) and reacted in a mixed solvent of a halogenated hydrocarbon and aqueous mixed solvent or a mixed solvent containing further an alcohol in the presence of an acid to give a complex. The resultant mixture consists of two layers of an aqueous layer and the halogenated hydrocarbon layer, and the d- and l-isomers without forming the complex are separated from the aqueous layer to separate the d- and l-isomers from the complex dissolved in the halogenated hydrocarbon. Thus, the optical resolution of the aimed benzothiazepine derivative expressed by formula II is carried out.