We report a case of fulminant type 1 diabetes in an 87-year-old man. The patient was admitted to our hospital because of hyperglycemia (blood glucose level was 712 mg/dl) and found to be in metabolic acidosis based on arterial blood gas analysis. We made a diagnosis of diabetic ketoacidosis, and instricted intensive insulin therapy. His hemoglobin A₁c level was 7.5%, and he had never been diagnosed with diabetes. No glutamic acid decarboxylase (GAD) antibody was detected in the serum, and the urinary C-peptide (CPR) level was below the level of detection. Serum CPR was still not detected after intravenous administration of 1 mg of glucagon. These findings were consistent with a diagnosis of fulminant type 1 diabetes mellitus. Our patient is the oldest patient with fulminant type 1 diabetes ever reported in Japan.