We report a case of 51-year-old woman with pure unroofed coronary sinus without persistent left superior vena cava and other cardiac anomaly. She presented with dyspnea on exertion during several years. Her chest film showed prominent cardiomegaly and dilated hilar vessels. Cardiac rhythm was atrial fibrillation. Transthoracic echocardiography demonstrated the enlarged coronary sinus with defect toward left atrium on parasternal long axis view and significant flow from coronary sinus into right atrium on subxyphoid view, and its other findings were dilated right ventricle and right atrium, paradoxical septal motion, moderate tricuspid regurgitation and mild mitral regurgitation, which were mimicking of large secundum atrial septal defect. Radionuclide cardioangiography and cardiac catheterization showed the existence of significant shunt. There was no evidence of persistent left superior vena cava on chest CT. Closure of Coronary sinus opening was done. Thereafter her symptoms of congestive heart failure were much improved. We think that careful examination of 2-D echocardiography can be valuable tool for diagnosis of unroofed coronary sinus in adult patient.