A 63-year-old male was admitted to our hospital with diabetic ketoacidosis. He had flu-like symptoms 10 days before admission and developed thirst, polyuria and anorexia with 9 kg of body weight loss in a week. Plasma glucose level on admission was 983 mg/dL and HbA1c was 7.5%. Despite high levels of serum pancreatic enzymes, lack of severe abdominal pain and no morphological change of pancreas in the abdominal CT scan eliminated the complication of classical acute pancreatitis. These findings suggested the diagnosis of fulminant type 1 diabetes. However, urinary and plasma C-peptide levels showed that insulin secretion was not completely depleted at onset. Furthermore, an examination of islet-related antibodies revealed the presence of high titer anti-GAD antibody. His HLA typing showed that DRB1*0901-DQB1*0303 and A24 were present. He has been doing well with continuation of insulin therapy. Over two years after onset, his plasma C-peptide level was gradually lowered, and anti-GAD antibody was still positive. Taken together, this is a rare case of abrupt onset autoimmune type 1 diabetes with transient but apparent exocrine pancreatic impairment at onset. Similar cases should be accumulated to clarify pathophysiological similarities and/or differences between fulminant type 1 diabetes and abrupt onset autoimmune type 1 diabetes.