PURPOSE: To obtain a (+) form of a benzotiazepine derivative having improved vasodilating action on a coronal blood vessel easily efficiently in high purity and in high yield, by using inexpensively obtainable optical active mandelic acid as a resolution agent, subjecting a specific benzothiazepine derivative to optical resolution.

CONSTITUTION: (±)-α-( p-Methoxyphenyl )-3-acetoxy-5-( β-dimethylaminoethyl )-2, 3-dihydro-1,5-benzothiazepine-4(5H)-one[(±)BTA for short]shown by the formula is reacted with preferably equimolar (+) or (-) mandelic acid (MA for short) as a resolution agent in a solvent, and, if necessary, a small amount of a seed of an optical active BTA.MA salt is inoculated to it and an optical resolution is carried out. The same kind salt as the precipitated (+)BTA.(+) MA salt or (-)BTA.(-)MA salt is separated. The resultant substance is reacted with a base such as NaOH, etc. so that the salt is resolved, to give a (+) form of benzothiazepine derivative shown by the formula or a (-) form shown by the formula having an arousal action on brain wave.