We report an extremely rare case of primary pancreatic plasmacytoma. A 56-year-old man had a 4-cm mass in the pancreatic tail and received distal pancreatectomy. This mass mainly consisted of plasma cells, but we failed to demonstrate their monoclonality in spite of the immunohistological staining. One and a half years later, this patient's right inguinal node swelled, and this node also showed a dense plasma cell infiltration. A very precise immunohistological staining was performed for this lymph node and the previous pancreatic mass, and both were diffusely positive for kappa light chain, IgG, and CD38. In the absence of myeloma elsewhere, we thus reached the correct diagnosis of primary pancreatic plasmacytoma, which later metastasized to lymph nodes. In the presence of the plasma cell proliferation in a pancreatic mass, plasmacytoma should be taken into consideration, and a more careful immunohistological staining is definitely necessary.