A 30-year-old man who had a painless mass in the right scrotum since 5 years ago underwent biopsy followed by right high orchiectomy. The tumor originating from the epididymis was histologically diagnosed as a malignant lymphoma (a diffuse large B-cell lymphoma). The tumor grew very slowly even though the MIB-1 index was more than 90%. The patient underwent six courses of R-CHOP chemotherapy, intrathecal administration of methotrexate, and radiation therapy to the contralateral testis. Currently, 10 months after surgery, the patient has shown no clinical evidence of recurrence.