We recently encountered a 66-year-old Japanese man who had suffered from acute hyperglycemia following flu-like symptoms during treatment of type 2 diabetes. Despite significantly increased plasma glucose levels, HbA1c was only slightly elevated. The possibility of autoimmune type 1 diabetes was excluded because of negative islet-related autoantibodies. Serum levels of pancreatic exocrine enzymes, amylase, lipase, and elastase-l were elevated. However, the insulin-secreting function of his islets was not severely damaged. This case is particularly notable for two reasons. First, it showed a fulminant type 1 diabetes-like clinical onset, but his beta cell function was fairly preserved. Second, it developed during the treatment of type 2 diabetes in an elderly patient.