PURPOSE: To obtain advantageously the titled substance which is an agent having coronary vasodilator action, etc., by reacting a benzothiazepine derivative with N-dimethylaminoethyl chloride in methyl ethyl ketone in the presence of pottasium hydrogencarbonate.

CONSTITUTION: A benzothiazepine derivative expressed by formula I is reacted with N,N-dimethylaminoethyl chloride as raw materials in methyl ketone in the presence of potassium hydrogencarbonate at a temperature of refluxing to afford the aimed compound expressed by formula II. Preferably, the potassium hydrogencarbonate is used in a molar amount of 2W3 times that of the compound expressed by formula I , and the compound expressed by formula I and the potassium hydrogencarbonate are suspended in the methyl ethyl ketone. N,N- dimethylaminoethyl chloride is preferably added to the suspension under agitation to carry out the reaction for 10W15hr.