We performed septal myectomy and mitral valve replacement successfully to a patient with hypertrophic obstructive cardiomyopathy complicated with mitral valve stenosis and regurgitation. Unexpected paroxysmal atrial fibrillation repeated since 2nd postoperative day and various kinds of class I oral antiarrythmic drugs were administered, to obtain normal sinus rhythm. However, on 9th postoperative day, the patient fell into pulssless ventricular tachycardia and resultant syncope and hemodynamic deterioration. Immediate cardiopulmonary resuscitation was started. Electrical cardioversion, intravenous infusion of disopyramide and verapamil were not effective but consecutive intravenous infusion of nifekalant succeeded to restore sinus rhythm. Nifekalant, a new class III antiarrythmic drug, is expected to be useful for lethal ventricular arrythmia in patients with hypertrophic obstructive cardiomyopathy.