: An 80-year-old Japanese woman had shown no indication of diabetes but regularly saw a primary-care physician for health management. Six months before her referral to our hospital, her HbA1c was 6.0%. She was referred to us for diabetic ketosis because she was urine ketone body-positive with a blood glucose level of 397 mg/dL and HbA1c of 14.6%. She was diagnosed with type 1 diabetes mellitus (T1DM) with glutamic acid decarboxylase (GAD) antibodies >2,000 U/mL (by ELISA) and IA-2 antibodies >30 U/mL. Insulin injections were introduced, and she was discharged. Laboratory tests during her hospitalization were negative for thyroid antibodies (TgAb, TPOAb). Elderly individuals with first-onset T1DM who are positive for IA-2 antibody are rare, and multiple-positive cases of pancreatic islet-associated autoantibodies are particularly rare. IA-2 antibodies have an approx. 60% positive rate in acute-onset T1DM, but they are more likely to be positive in children and adolescents and are known to turn negative earlier than anti-GAD antibodies. Although a large amount of insulin is needed in general in such cases, our patient was successfully treated with a small amount of insulin. IA-2 antibody has been reported to be positive even in GAD antibody-negative individuals. In some cases, IA-2 antibody and other antibodies are positive even in elderly-onset diabetes, and this contributes to the diagnosis of T1DM.