Idiopathic spontaneous intraperitoneal hemorrhage is a rare, often fatal condition; however, the patients present with nonspecific symptoms. Even after a comprehensive radiological examination and surgical exploration, the origin of hemorrhage often remains unclear. A 74-year-old woman with sudden abdominal pain, nausea, and vomiting was admitted to our hospital. She had no history of trauma. Computed tomography showed a huge hematoma along the small intestinal mesentery without evidence of visceral organ abnormality causing bleeding. The hematoma was completely removed by surgery and no origin of hemorrhage was detected. Herein, we report on a case of idiopathic spontaneous intraperitoneal hemorrhage.