Omental torsion is a rare cause of acute abdomen and sometimes requires surgery. Recently, we encountered a case of omental torsion diagnosed as omental infarction preoperatively. An 18-year-old male presented to our emergency room with a chief complaint of lower abdominal pain since previous 2 days. Because of his history of Down syndrome, an abdominal examination was very difficult. Plain abdominal computed tomography (CT) suggested omental hernia adhering to the right paracolic gutters. Two days after hospital admission, symptoms did not improve, and contrast-enhanced abdominal CT suggested omental infarction. We performed an emergency surgery. Upon exploration of the abdominal cavity, the greater omentum was found to be twisted four times and adhered to the right paracolic gutters. We performed a partial omentectomy. He was discharged 9 days after the surgery. There was no cause of omental torsion in the abdominal cavity, and he was diagnosed as having idiopathic omental torsion. In cases wherein the cause of acute abdomen cannot be detected, omental torsion should be considered, and abdominal CT could be helpful for the diagnosis.