A 60-year-old obese woman was admitted to our hospital to receive surgical treatment for "slow-growing mediastinal lipoma". With the non-invasive examination such as CT scan and ultrasonography, she was revealed as Morgagni's hernia containing a greater omentum. With a trans-abdominal approach, we could take a good surgical field for the bilateral foramina of Morgagni. Several discussions were described for availability of CT scan, ultrasonography and a trans-abdominal approach.