A 50-year-old man was admitted to our hospital for the evaluation of a blunted left costophrenic angle on chest roentgenograms. Chest non-enhanced computed tomographic (CT) scans demonstrated the omentum as a crescent-shaped, fat-dense, mass-like shadow on the left dorsal thoracic wall, contiguous with the abdominal cavity and floating. Omental vessels were recognized as funicular water densities running in the superior-to-inferior direction. Magnetic resonance imaging disclosed that the omental vessels ran continuously from the abdominal cavity into the thoracic cavity through a defect in the diaphragm. Diaphragmatic hernia was diagnosed, and corrective surgery performed. We reported this case because the CT-based delineation of omental vessels protruding into the thoracic cavity is rare.