We herein report a case of anaplastic carcinoma of the pancreas arising in an intraductal papillary mucinous neoplasm (IPMN). A 68-year-old Japanese woman was admitted to our hospital complaining of fatigue. Computed tomography revealed an irregular mass in the pancreatic head, which displayed high-signal intensity on diffusion-weighted magnetic resonance imaging. Accordingly, the patient was diagnosed with pancreatic cancer and underwent pancreaticoduodenectomy. The histopathological findings revealed intraductal papillary proliferative changes involving the main and branch ducts of the pancreatic head. Based on the immunohistochemistry results, the intraductal lesion was diagnosed as IPMN. The pathological diagnosis for the invasive carcinoma was anaplastic giant-cell carcinoma of the pancreas (ACP), and the focus of IPMN dedifferentiation to ACP was found to be located at the periphery of the IPMN. At 18 months postoperatively, the patient remains disease-free.