Duodenal benign tumors are relatively rare. However, owing to the development of endoscopic ultrasonography (EUS), reports about these cases have been increasing. In this report, we cite a case of Brunner's cyst of the duodenum, in which EUS proved very useful in making the diagnosis. Patient: a 41-year-old man was admitted to our hospital for further examination of a duodenal tumor. Upper GI series and endoscopy revealed a polypoid tumor in the second portion of the duodenum. Using EUS, we located the tumor in the third layer of the duodenal wall, which corresponded to the submucosal layer, and had a so-called mixed (solid and cystic) pattern. Polypectomy was performed by endoscopy, and Brunner's gland hyperplasia was histologically proved.