We report a duodenal gastrinoma in a 50-year-old man who was admitted to our hospital with tarry stools. Esophagogastroduodenoscopy revealed multiple ulcers in the duodenal bulb and a submucosal tumor in the descending duodenum. His serum gastrin level was 1400pg/ml. We suspected Zollinger-Ellison syndrome and performed selective arterial calcium injection to locate the gastrinoma. Increase in the hepatic venous gastrin level was seen only in the gastroduodenal artery area. We diagnosed a gastrinoma located in the pancreaticoduodenal area. Genetic examination showed a single-base deletion in the MEN-1 gene. At operation, the tumor was found in the submucosal layer of the descending duodenum and was extirpated. He is alive without recurrence 3 years after surgery.