The present invention provides: a novel compound which is characterized by being capable of increasing the number of heart beats or a blood pressure mildly to improve hemodynamics and which is useful as a therapeutic or prophylactic agent for atrial fibrillation and heart failure and a pharmaceutical composition which contains the compound. The present invention relates to: an optical isomer of a 1 4 benzothiazepine 1 oxide derivative represented by general formula [II] (wherein R represents a hydrogen atom or a hydroxy group and indicates the presence of an optical isomer) or a pharmaceutically acceptable salt thereof and a pharmaceutical composition which contains the optical isomer or a pharmaceutically acceptable salt thereof.
