Neuromyelitis optica (NMO) is a relapsing inflammatory disease of the central nervous system, usually affecting the optic nerves and the spinal cord. It is presumed to be an antibody-mediated disorder and the target antigen is the water channel aquaporin-4 (AQP4) on astrocyte cell membranes. NMO is a disease caused by astrocyte disorder and is distinct from multiple sclerosis (MS), which is a primarily demyelinating disease caused by oligodendrocyte disorder. In NMO, spinal MRI shows a T2-hyperintense, longitudinally extensive (≥ 3 vertebral segments) spinal cord lesion. The case, which has optic neuritis or transverse myelitis with the presence of AQP4 antibody, is called as NMO spectrum disorder. A 68 year-old woman with a history of hypertension and diabetes mellitus was brought to the former hospital by ambulance with acute onset of tetraparesis. She denied visual acuity disturbance. MRI revealed a T2-hyperintense lesion from C5 to T2 level. Laboratory examination showed the presence of AQP4 antibody and the absence of oligoclonal bands. Low-dose steroid treatment was started after establishing a diagnosis of NMO. She incompletely recovered from disability, although the T2-hyperintense lesion on MRI had almost disappeared six months after the onset. It is important to maintain a high index of suspicion for NMO in cases with a longitudinally extensive spinal cord lesion, because untreated NMO leads to severe disability.