A 57-year-old man with fatigue was admitted to our hospital. Abdominal contrast computed tomography indicated the presence of a 35mm tumor in the pancreatic head and dilation of the main pancreatic duct from its body to the tail. Endoscopic ultrasonography revealed that the tumor had infiltrated and occupied the main pancreatic duct, and endoscopic retrograde pancreatography confirmed that the tumor was present in the main pancreatic duct. Tumor biopsy via endoscopic retrograde cholangiopancreatography demonstrated the proliferation of spindle and pleomorphic cells. Therefore, the patient was diagnosed with anaplastic pancreatic carcinoma and underwent subtotal stomach-preserving pancreaticoduodenectomy. Histological analysis showed the prevalence of adenocarcinoma and anaplastic carcinoma cells in the pancreatic parenchyma and main pancreatic duct, respectively. Anaplastic carcinoma cells showed a decrease in E-cadherin staining. In conclusion, tumor cell proliferation and lack of cell adhesion may have caused the infiltration into the main pancreatic duct.