Fourty seven years old woman came to our hospital for further examination of incidentally found abnormal chest shadow. Chest US examination revealed lobulated cystic mass. The cyst wall was thin and smooth. Chest computed tomography showed water density cystic mass. Preoperative diagnosis was pericardial cyst. Operation was done. Lobulated cyst was attached to pericardium and diaphragma. Though adhesion to the pericardium was loose, adhesion to the diaphragma was tight. To achieve complete resection of the cyst, partial resection of the diaphragma was needed. Postoperative course was uneventful. Cystic lymphangioma is benign but complications such as infection or bleeding were reported. Then complete resection of the cyst should be done.