A 48-year-old woman was admitted to our hospital because of abnormal shadow on the chest X-ray. Chest contrast CT scan showed roundly mass in the posterior mediastinum which were combined with and without contrast elements, and chest MRI (T2 weighted) showed high signal intensity. These features suggested mediastinal cyst or extralobar sequestration. The operation was performed through left 6th intercostal thoracotomy. Two different lesions connected to the mediastinum were confirmed, a cystic tumor and small accessory lung. The former was diagnosed as bronchogenic cyst and the latter as extralobar pulmonary sequestration.