A 55-year-old male was referred to our hospital with the chief complaint of painless right scrotal swelling. Serum tumor marker levels were all within the normal range. A right radical orchiectomy was performed and the histopathological diagnosis was plasmacytoma. Chest and abdominal computed tomographic scan and bone scintigraphy did not show metastasis. Serum and urine protein electrophoresis and bone marrow biopsy were negative. Diagnosis was primary testicular plasmacytoma (extramedullary plasmacytoma). Eleven months after the operation, multiple bone lesions appeared and our diagnosis was conversion to multiple myeloma.