A 52-year-old woman underwent esophagogastroduodenoscopy after an abnormal medical examination, which revealed a mass lesion over half the circumference of the superior duodenal angulus. Immunostaining was diffusely positive for somatostatin, synaptophysin, and chromogranin A. A 3 cm-sized mass in the pancreaticoduodenal region and multiple nodular lesions of a few mm in both lobes of the liver were revealed by CT. The diagnosis is primary somatostatin-producing tumor of the duodenum with multiple liver metastases. She underwent gastric jejunal bypass for impaired transit. Afterwards hepatic infusion and systemic chemotherapy were continued, and 5 years passed without progression. When she stopped chemotherapy for 6 months, she started somatostatin analogue therapy because of the increase of the tumors. The tumors did not increase, and 20 years have passed since the start of treatment. We report a case of primary somatostatin-producing tumor of the duodenum with liver metastases that is still alive for a long period of time, with a review of the literature.