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.
