We report a case of concomitant pancreatic endocrine neoplasm (PEN) and intraductal papillary mucinous neoplasm (IPMN). A 74-year-old man had been followed-up for mixed-type IPMN for 10 years. Recent magnetic resonance images revealed an increase in size of the branch duct IPMN in the pancreas head, while the dilation of the main pancreatic duct showed minimal change. Although contrast-enhanced computed tomography and magnetic resonance imaging did not reveal any nodules in the branch duct IPMN, endoscopic ultrasound indicated a suspected nodule in the IPMN. A malignancy in the branch duct IPMN was suspected and we performed pylorus-preserving pancreatoduodenectomy with lymphadenectomy. The resected specimen contained a cystic lesion, 10 x 10 mm in diameter, in the head of the pancreas. Histological examination revealed that the dilated main pancreatic duct and the branch ducts were composed of intraductal papillary mucinous adenoma with mild atypia. No evidence of carcinoma was detected in the specimen. Incidentally, a 3-mm nodule consisting of small neuroendocrine cells was found in the main pancreatic duct. The cells demonstrated positive staining for chromogranin A, synaptophysin, and glucagon but negative staining for insulin and somatostatin. Therefore, the 3-mm nodule was diagnosed as a PEN. Since the mitotic count per 10 high-power fields was less than 2 and the Ki-67 index was less than 2%, the PEN was pathologically classified as low-grade (G1) according to the 2010 World Health Organization (WHO) criteria. Herein, we review the case and relevant studies in the literature and discuss issues related to the synchronous occurrence of the relatively rare tumors, PEN and IPMN.