A 49-year-old man was admitted suffering from headache persisting for a month. He had a history of primary spinal intramedullary malignant melanoma at the T6 level 18 years previously, which had remained stable for 18 years. Magnetic resonance imaging revealed central nervous system (CNS) dissemination of malignant melanoma. Whole brain radiation therapy (30 Gy), local radiation therapy (15 Gy), and routine intrathecal injection of interferon beta were performed. The progression of CNS dissemination of malignant melanoma was controlled without neurological deterioration for 38 months. The prognosis for primary CNS malignant melanomas better than that for cutaneous melanoma. However, the clinical course is still unknown, and CNS dissemination is regarded fatal. The unusually long survival in the present case indicates the effectiveness of the combined radiotherapy and interferon therapy.