A 79-year-old man was admitted to our hospital because of swallowing disturbance. Chest X-ray and computed tomography (CT) scan revealed 7 x 6 cm cystic shadow in posterior mediastinum. We diagnosed that swallowing disturbance caused by pericardial cyst. Cysticotmy was performed. Three days after operation, chylothorax occurred. Conservative therapy was not effective, we performed re-operation 28 days later from the 1st operation. There was aperture of thoracic duct inside of cyst, and thoracic duct was ligated. After the 2nd operation, chylothorax was cured.