A woman in her seventies visited a previous doctor because she was reported to have abnormal opacity in her right lung. Computed Tomography (CT) revealed pulmonary tuberculosis. An upper abdominal mass was also found, and she was referred to our hospital. Contrast-enhanced abdominal CT showed a heterogeneously mass lesion with a long diameter of approximately 83 mm in the pancreatic caudal region. Endoscopic ultrasound revealed a tumor at the pancreatic tail with distinct boundaries and internal mosaic echo. Magnetic Resonance Imaging showed a mass with heterogeneous and partially cystic degeneration. Based on these findings, solid pseudopapillary neoplasm (SPN) and other malignant lesions of the pancreas tail region were suspected. By laparotomy with subarachnoid arc incision, distal pancreatectomy, splenectomy, and regional lymph node dissection were performed. The specimen was a cystic mass covered with an 80×75×65 mm membrane in the pancreatic tail region. Initially, in the results of the pathological diagnosis, epithelial cells were not observed, and a hematoma was diagnosed. However, subsequent diagnosis indicated bizarre idioblasts and numerous osteoclast-like giant cells in the surrounding capsule. This area was initially thought to be a reactive pseudotumor lesion due to hemorrhage; however, the presence of CK7, p53-positive atypical spindle-shaped tumor cells and polynucleated tumor cells led to the diagnosis of anaplastic pancreatic carcinoma with osteoclastic polynucleic giant cells with extensive necrosis. We report a case of a rare pancreatic tumor with extensive intratumoral necrosis, which was difficult to diagnose pathologically, and present our findings with literature review.