Primary malignant lymphoma of the cauda equina is an extremely rare disease. Previously, there have been only 12 reported cases of malignant lymphoma of the cauda equina, and most cases relapsed early in the clinical course. So, the optimal treatment strategy for this condition has not been established yet and the prognosis is thought to be poor. We experienced a case of B-cell malignant lymphoma of the cauda equina, with rapid progression of the muscle weakness of the bilateral lower extremities, successfully treated with high-dose methotrexate plus high-dose cytarabine (Ara-C) chemotherapy, followed by radiotherapy and in complete remission without any recurrence signs, 1.5 years after the initial diagnosis. Intrathecal chemotherapy with MTX, Ara-C, and predonisolone was simultaneously performed. We should carefully continue to monitor the clinical course of our case, with the examinations of magnetic resonance imaging and cerebrospinal fluid in order not to overlook any subtle neurological changes or other clinical symptoms.