A 64-years-old man referred to a hospital because of high-grade fever. He was diagnosed as having influenza B by "POCTEM Influenza A/B", a rapid influenza diagnostic kit which detect some antigens of influenza virus. Six days after medication of oseltamivir phosphate, his flu-symptoms disappeared, but he complained sever thirsty. And after 2days, he suffered from loss of consciousness and was admitted to the hospital. Laboratory data on admission showed diabetes ketoacidosis, slight elevation of HbA1c level despite sever hyperglycemia, and increase of serum amylase concentration. Anti GAD antibody and anti IA-2 antibody were not detected. Urinary C-peptide excretion was undetectable and serum C-peptide levels were also undetectable after glucagon and arginin load, suggesting disappearance of endogeneous insulin secretion. Class II HLA was susceptible to fulminant type1 diabetes. Based on these findings, we diagnosed him with fulminant type1 diabetes. In Japan, only three viruses in three cases have been reported to be the trigger in the development of fulminant type 1 diabetes. They were human herpes virus 6, herpes simplex virus and Coxsackie B3 virus. This is the fourth report of fulminant type 1 diabetes developed after the established diagnosis of viral infection and the first after influenza B virus infection. The fact that fulminant type 1 diabetes developed after the infection of such a common virus suggest that factors within host will play more important roles than virus itself in the etiology of fulminant type 1 diabetes.