Idiopathic perforation of the gallbladder without cholecystitis, tumor or injury, is extremely rare and only 25 cases of this disorder have been reported in the Japanese literature. An 80-year-old woman was admitted to the hospital because of abdominal pain. Computed tomography revealed massive ascites and swelling of the pancreatic head. She was diagnosed as having terminal stage cancer of the pancreatic head. Because of Increasing ascites volume, percutaneus abdominal drainage was carried out. Biliary ascites was drained. After improvement of her general condition, ERCP was carried out. As ERCP showed no pancreatic lesion, operation was carried out. Perforation was found in neck of the gallbladder. Cholecystectomy was carried out. No inflammatory or oncogenic lesion was observed on the surgical specimen. Histologically, no venous thrombosis was found, and cell infiltration was mild.