Japanese-B viral encephalitis (JE) usually has a monophasic illness pattern. A 45-year-old woman in an altered mentality had improved over 1 month. About 1 week after the initial improvement, the patient became comatose with aggravated EEG and MRI findings. Assays of cerebrospinal fluid and serum were positive for the IgM antibody to Japanese-B virus. After intravenous immunoglobulin (IVIG) infusion, the patient recovered. We report a patient with JE who showed a biphasic illness pattern and recovered after IVIG therapy.