A 71-year-old man presented with gradually progressing cognitive decline following acute febrile exanthematous disorder. The MRI showed an abnormality in the bilateral limbic systems. An elevation of cerebrospinal fluid (CSF) protein with lymphocyte pleocytosis was noted. Immunoblot of the CSF revealed the presence of anti-white matter antibodies that mainly recognized astrocytes. Intravenous steroid followed by oral steroid reduced the symptoms to a remarkable degree. The patient has now been successfully sustained with steroid for more than two years. We considered that this case is classified as non-paraneoplastic limbic encephalitis, and acquired autoimmunity played a major role in the pathogenesis of this case.