A 78-year-old man was referred to our hospital owing to a pancreatic tumor detected on ultrasonography. He showed weight loss, and his diabetes mellitus had worsened over 3 months. Abdominal contrast-enhanced computed tomography revealed a 3 cm diameter pancreatic head tumor with peripheral enhancement and a dilated pancreatic duct. He underwent subtotal stomach-preserving pancreaticoduodenectomy. Examination of intraoperative frozen sections did not indicate cancer involvement at the edge of the resected pancreatic duct. The tumor was histologically diagnosed as mixed acinar-neuroendocrine carcinoma(MANEC). It was mainly detected in the pancreatic head and was accompanied by intra-ductal growth in the pancreatic duct. The patient has survived for 21 months without recurrence. MANEC is a rare disease associated with intraductal growth. Intraoperative diagnosis of tumor involvement at the edge of the pancreatic duct may be useful for R0 resection in surgery for MANEC.