PURPOSE: To prepare an optically active benzothiazepine derivative useful as an antidepressant, tranquilizer, and a coronary vasodilator, by subjecting a recemic modification, an intermediate, to optical resolution with an optically active organic base or organic acid, followed by ring closure.

CONSTITUTION: A racemic modification shown by the formula I is subjected to optical resolution with an optically active organic base (e.g., quinine, ephedrine, etc.) or an optically active organic acid (e.g., carbobenzoxyalanine ,dibenzoyl- d-tartaric acid, etc.) to give an optically active substance, which is subjected to ring closure with acetic anhydride to give optically active α-2-(4-methoxyphenyl)-3-acetoxy-5-( 2-dime thylaminoethyl )-2, 3-dihydro-1,5-benzothiazepine-4(5H)-one shown by the formula II and its salt. Or, a racemic modification shown by the formula III is similarly subjected to optical resolution, and to ring closure to give a compound shown by the formula II.