A 70-year-old man with aphasia due to ischemic cerebral events by thromboemboli was admitted. The cause of thromboemboli was investigated, and transesophageal echocardiography and chest MRI revealed an encapsulated cystic mass in the right atrium, not circulating. It was a homogeneous cystic mass suggesting a tumor (Myxoma) rather than thrombus. Right atrial mass was resected together with partial atrial septum under the normgthermic cardiopulmonary bypass. Histologically it was an atrial septal aneurysm, closed on itself, filled with blood. We re-port this rarely seen case with a review of the literatures.