A 58-year-old man was brought to our hospital with left upper abdominal pain which suddenly appeared on the previous evening. An abdominal CT scan showed localized retention of ascites, a slightly high density mass under the left upper abdominal wall, with a high density area detected within the mass, which was suggestive of leakage of contrast medium from peripheral branches of the omental artery. From these findings intraperitoneal hemorrhage caused by bleeding from the greater omentum was suspected. Angiographic examination of the abdomen indicated extravasation of contrast medium from blood vessels of the right gastroepiploic artery. Transarterial embolization was carried out and permanent hemostasis was achieved. Injury, anticoagulant, neoplasms, varix, torsion of the omentum, and segmental arterial mediolysis (SAM) etc have been reported as causes of omental bleeding, but none of these were found in our case. We diagnosed the present case as idiopathic omental bleeding.