An inferior middle mediastinal mass was observed in a 51-year-old woman initially presenting as pericardial effusion and mimicking lymph node disease. Thoracotomy demonstrated a tumor invading the posterior atrial wall. Complete resection was possible under the extracorporeal circulation. The tumor proved to be a soft tissue myoepithelioma in an exceptional location. Despite the difficult surgical resection due to location, prognosis of this absolutely rare tumor without cytologic features of malignancy should be good.