A 45-year-old woman presented with a rare case of metastatic cauda equina tumor from breast cancer without spinal column or brain metastasis. She had undergone resection of breast cancer 4 years previously. She presented with a 2-month history of severe low back pain. Magnetic resonance imaging showed a well-enhanced intradural extramedullary mass at the L1 level without other intradural lesions. At surgery, the tumor was partially removed to preserve the nerve root function under electrophysiological monitoring. The histological diagnosis was adenocarcinoma. The tumor was located in the subarachnoid space, suggesting hematogenous metastasis from the breast cancer. Postoperatively the pain subsided and no neurological deficit occurred. She underwent adjuvant therapy and rehabilitation. Cauda equina tumors may be relatively progressive regardless of imaging findings and clinical symptoms, so preoperative systemic investigation should be conducted, considering the possibility of metastatic tumor. A comprehensive therapeutic strategy involving adjuvant therapy after surgery is important to establish, considering the preservation of postoperative nerve function.