A 28-year-old man whose chest X-ray film showed a mass on the right upper mediastinum was admitted. Preoperative examination, including CT, MRI, esophagogram, did not make it possible to determine clinically from which organ the tumor originated. On diagnosing with transesophageal ultrasonic endoscopy (TUE) the tumor was found to originate in the esophageal submucosa, with no involvement of the mediastinal organ apparent. Esophageal sub-mucosal tumor enucleation was performed. The resected tumor was 15 cm in long diameter and weighed 125 g. We therefore believe that TUE is clinically useful in diagnosing the tumor originating in the esophageal wall or adjoining esophageal wall.