PURPOSE: To profitably and simply obtain the intermediate of diltiazem hydrochloride having a coronary vasodilating action and useful for treating ischemic cardiac diseases simply in one process and in a good yield without requiring any prelimanary treatment.

CONSTITUTION: (A) A phenylpropionic acid derivative of formula I (R is methyl, ethyl) is reacted with (B) N,N-dimethylaminoethyl chloride or its acid salt in an amount of equimolar or slightly excessive mole, preferably 1-2 times mole, in the presence of an alkali metal hydroxide, especially potassium hydroxide, in an amount of 1-5 times mole, especially 1.3-3 times mole, in an etheric solvent (preferably a cyclic ether such as dioxane or tetrahydrofuran) preferably at approximately 50°C; to the boiling point of the solvent to advantageously obtain 5-[2-(dimethylamino)-ethyl]-3-hydroxy-2-(4-methoxyphenyl)-2,3-dihydro-1,5- benzothiazepine-4 (5H)-one of formula II useful as the intermediate of medicines.