A 71-year-old man was admitted due to brain infarction. An abnormal floating mass was found in the right atrium by echocardiography. The mass was assumed to be a thrombus. The patient was failed to be in danger of pulmonary embolism, so it was decided to remove surgically. The thrombus was removed under cardiopulmonary bypass including the atrial wall where it was attached, because it was difficult to rule out the possibility of cardiac tumor macroscopically. The tumor showed pathological findings of thrombus. Postoperative course was uneventful.