PURPOSE: To produce the titled compound having excellent antiulcer activity and gastric juice secretion suppressing activity, in high yield, by reacting a benzothiazepine derivative with N-methylpiperazine under heating in the presence of a trans-compound produced as a by-product.

CONSTITUTION: The benzothiazepine derivative of formula II [e.g. 2,3-dihydro- cis-3-(4-methylpiperazinylmethyl)-2-phenyl-1,5-benzothiazepin-4(5H)-one, etc.] is produced by the thermal reaction of the benzothiazepine derivative of formula I (X is acid residue such as Cl, methanesulfonyloxy, p-toluenesulfonyloxy, etc.; R is H, methyl, Cl, methoxy, etc.) with N-methylpiperazine. The reaction is carried out by adding preferably a physically practicable amount of the trans-compound of formula III which is produced as a by-product of the above reaction at about 100W140°C. The compound of formula III has different properties from those of the compound of formula II, and accordingly, it can be recovered easily and reused in the next reaction.