A 57-year-old man was admitted to our hospital complaining of poor appetite. He had been diagnosed with diabetes mellitus and was anti-GAD antibody (GAD-Ab) negative 1 year previously, at the age of 56 years old. Abdominal CT revealed pancreas tail swelling; elastase-I level was elevated and he was diagnosed with pancreatitis. The level of GAD-Ab was increased and HLA haplotype was DRB1*0901-DQB1*0303, which is seen frequently in type 1 diabetic Japanese patients. However, his endogenous insulin secretion ability was not deteriorated. After elastase-I level and pancreas swelling improved, GAD-Ab returned to a normal range. One year after the onset of pancreas swelling, he was still not in an insulin-dependent state. In this case, transient GAD-Ab positivity with susceptible haplotype for type 1 diabetes mellitus might have been induced by a GAD antigen discharged from the destroyed islet due to pancreatitis.