Although metastatic lung adenocarcinoma in the spinal cord is rare, it can be diagnosed by positron emission tomography and computed tomography (PET/CT) scan with high sensitivity during the early disease stage. A clinical and radiographic review was performed to present a rare case of an intradural intramedullary adenocarcinoma metastasis in the spinal cord with a negative PET/CT scan. A 75-year-old man with a diagnosis of lung cancer without metastasis confirmed by a negative PET/CT scan with no spinal symtoms (conducted 6 weeks previously) presented with progressive paralysis of both lower extremities and accompanying bowel and bladder symptoms. He underwent radical lobectomy of left lung under diagnosis of lung cancer without distant metastasis 6 weeks ago. Emergent MRI was performed, and MRI revealed a large intradural intramedullary mass compressing the spinal cord and extending from T12 to L1 with anterior compression of the spinal cord. Surgical decompression and tumor resection from T12 to L1 by lumbar laminectomy and durotomy were performed under a microscope. And, a diagnosis of adenocarcinoma to the spinal cord was made based on histopathologic findings. Postoperatively, the patient’s neurologic status was not significantly improved. Despite a negative PET/CT scan finding with no neurologic symptoms or pain, surgeons should not exclude the possibility of a spinal metastatic lesion with lung cancer.