A 74-year-old woman with a history of a benign gastric polyp was admitted to our hospital with a complaint of epigastric discomfort. A diagnosis of primary gastric lymphoma was confirmed by endoscopy and results of abrasive cytology. Endoscopic ultrasonography (EUS), performed prior to surgery, demonstrated clearly the submucosal invasion of the gastric lymphoma. Histopathology on the depth and the involvement of perigastric lymph nodes of surgical specimen confirmed the accuracy of the EUS findings. Thus EUS offers a means of early preoperative diagnosis and staging of gastric lymphoma, and that is superior to conventional endoscopy in such cases.