Inclusion Criteria:  Patients who visit the emergency department and in-patients over 18 years Severe hypernatremia: glucose-corrected serum sodium ≥ 155 mmol/L Written consent  Exclusion Criteria:  Arterial hypotension requiring inotropes or vasopressors (systolic blood pressure < 90 mmHg and mean arterial pressure < 70 mmHg) Anuria or bilateral urinary outlet obstruction Uncontrolled diabetes mellitus (HbA1C > 9%) or glucose at baseline > 500 mg/dL or diabetic ketoacidosis or hyperosmolar hyperglycemic syndrome Decompensated liver cirrhosis (LC) - Known LC with ascites or diuretic use or hepatic encephalopathy or varix End-stage renal disease receiving renal replacement therapy Women who are pregnant or breast feeding  Patients with the following conditions within 30 days prior to randomization:  History of cardiac surgery excluding PCA, acute myocardial infarction, sustained ventricular tachycardia, ventricular fibrillation, acute coronary syndrome, and admission for heart failure Uncontrolled increase of intracranial pressure The subjects judged by investigators to have difficulty continuing the trial were also excluded.