A 59-year-old Japanese man with a history of chronic hepatitis C and cirrhosis was admitted to hospital because of severe abdominal pain and diarrhea. His dis-comfort had begun 2 months earlier and was localized to the upper abdomen. Upper gastrointestinal endoscopy showed multiple ulcerative lesions from the duodenal bulb to the descending part of the duodenum, one of which was a giant ulcer that filled half of the intestinal lumen. Despite continuous intravenous lansoprazole therapy, his abdomi-nal symptoms did not improve. Upper gastrointestinal endoscopy was again performed to detect the tumor, but it was difficult to observe the tumor with a conventional endoscope. We then inverted a transnasal endoscope into the duodenum, and this enabled us to detect a 15-mm submucosal tumor in the upper wall of the duodenal bulb. Examination of specimens from endoscopic ultrasonogra-phy fine-needle aspiration biopsy of the tumor revealed gastrinoma in the duodenal bulb. We decided to perform an operative resection. The patient's symptoms resolved after surgery, and he remained asymptomatic at follow-up 18 months later. Therefore, when it is difficult to detect the tumor directly by conventional endoscopy, we recommend that attempts be made to detect the tumor by inverting a transnasal endoscope into the duodenal bulb.