We reported a rare case of mediastinal cystic lymphangioma, which happened to be discovered on chest X-ray. CT density level of the mediastinal tumor was 20 HU and the tumor was homogeneous cystic lesion. Multilocularity of the cyst and the relationship to the adjacent inferior vena cava were clarified by pre-operative transesophageal and intraoperative ultrasonographic examination, and the tumor was successfully excised without mishap.