A 42-year-old man, after remission of MALT lymphoma of the small intestine, was repeatedly hospitalized because of abdominal pain and severe dehydration caused by frequent vomiting and watery diarrhea. His symptoms would improve quickly every time when he was fasted and inserted a nasogastric tube. We were unable to find abnormalities on endoscopic examination and computed tomography. He was suspected to have gastrinoma because of active bleeding from a duodenal ulcer. High-level serum gastrin, endoscopic ultrasound, somatostatin receptor scintigraphy, and selective arterial calcium injection test were done. He was diagnosed with pancreatic gastrinoma in the pancreatic head by endoscopic ultrasound fine needle aspiration and subsequently underwent pancreatoduodenectomy. Histopathologic findings showed a 3-mm neuroendocrine tumor located in the duodenal submucosal layer. The presence of metastasis was confirmed in one of the peripancreatic lymph nodes. The pancreatic gastrinoma in the pancreatic head that we initially diagnosed was a lymph node metastasis behind the pancreas. Because additional resection was performed on the duodenum, we were able obtain a diagnosis of duodenal gastrinoma.