Insulin autoimmune syndrome is one of the rare causes of hypoglycemia, and characterized by hyperinsulinemic hypoglycemia associated with high titer of antibodies to endogenous insulin. We report a case of insulin autoimmune syndrome in a 57-year-old woman, presenting with mental changes due to hypoglycemia. She had no history of diabetes or insulin administration. The serum C-peptide level was 4.69 ng/mL and the insulin concentration was 229.55 µU/mL, when fasting plasma glucose level was 32 mg/dL. The insulin-to-glucose ratio was 7.17, while there was no radiologic evidence of insulinoma. The insulin antibody level was over 100 µU/mL, resulting in the diagnosis of insulin autoimmune syndrome. Hormonal studies revealed partial hypopituitarism and a lack of glucagon-response to hypoglycemia. Hypoglycemia disappeared with replacement of prednisolone with levothyroxine therapy. Under secretion of growth hormone and of adrenocorticotropic hormone due to hypopituitarism were associated with insufficient counterregulation to hypoglycemia. One should keep in mind that insulin autoimmune syndrome or hypopituitarism is one cause of hypoglycemia in patients with no history of diabetes, and corticosteroid can be an effective treatment for both diseases.