Inclusion Criteria  < At 1st registration > Patients meeting the possible diagnostic criteria for autoimmune encephalitis < At 1st registration > Patients with a CASE score of 5 to 22 during the screening period < At 1st registration > Patients with autoimmune encephalitis in progress (active and requiring therapeutic intervention) < At 1st registration > IVIG therapy and steroid pulse therapy are considered necessary by the investigator. < At 1st registration > Patients aged 15 years or older at the time of informed consent  < At 2nd registration > Patients who meet any of the following (1) to (6):  Definite diagnostic criteria for autoimmune limbic encephalitis MRI evidence of demyelination (probable autoimmune encephalitis) Probabilistic diagnostic criteria for anti-NMDAR encephalitis Probabilistic diagnostic criteria for Bickerstaff brainstem encephalitis Probabilistic diagnostic criteria for Hashimoto's encephalopathy Diagnostic Criteria for Autoimmune Encephalitis with Negative but Probable Autoantibodies < At 2nd registration > CASE score of 5 to 22 on Day 8 of the previous treatment period < At 2nd registration > Patients who have had an inadequate response to steroid pulse therapy  Exclusion Criteria:  < At 1st registration > Patients with strongly suspected infectious encephalitis < At 1st registration > Patients who received immunoglobulin preparations within 8 weeks prior to informed consent < At 1st registration > Patients who received plasma exchange within 4 weeks prior to informed consent < At 1st registration > Patients who received immunosuppressants (Rituximab, cyclophosphamide, etc.) within 4 weeks prior to informed consent < At 1st registration > Patients who have had tumor resection associated with autoimmune encephalitis within 4 weeks prior to informed consent < At 1st registration > Patients with a history of shock or hypersensitivity to the ingredients of NPB-01 < At 1st registration > Patients with known IgA deficiency < At 1st registration > Patients with renal disorder < At 1st registration > Patients with a current or previous history of cerebral or cardiovascular disorders (Asymptomatic cerebral infarction and myocardial infarction that occurred more than 5 years ago are not applicable.) < At 1st registration > Patients at high risk of thromboembolism < At 1st registration > Patients with haemolytic/blood loss anaemia < At 1st registration > Immunosuppressed/immunocompromised patients < At 1st registration > Patients with decreased cardiac function < At 1st registration > Pregnant, expected (desired or planned) pregnant, or breastfeeding patients < At 1st registration > Use of prohibited medications or treatment in this study < At 1st registration > Patients who received investigational product in this study (re-enrollment prohibited) < At 1st registration > Patients who have received treatment with investigational product other than this study within 4 months prior to informed consent < At 1st registration > Patients with a history of hypersensitivity to methylprednisolone sodium succinate < At 1st registration > Patients who have a tumor associated with autoimmune encephalitis and are considered to require resection during the study period. < At 1st registration > Patients receiving intravenous general anesthetics or sedative hypnotics < At 1st registration > Patients in coma < At 1st registration > Ventilated patients < At 1st registration > Patients who cannot undergo protocol-specified tests/assessments < At 1st registration > Other patients considered ineligible for the study by the investigator < At 2nd registration > Positive herpes simplex virus DNA qualitative test in the screening period. < At 2nd registration > Serum creatinine ≥ 2 times the upper limit of normal during the screening period. < At 2nd registration > Total protein ≥ 9 g/dL during the screening period. < At 2nd registration > Patients with hematocrit ≥ 55% during the screening period < At 2nd registration > Patients who meet any of the exclusion criteria at the time of first registration