A 65-year-old woman presented to a hospital with complaints of dyspnea and lumbar pain. Chest computed tomography (CT) showed left pleural effusion. Thoracentesis showed pleural effusion with elevated levels of amylase. Enhanced CT showed fluid accumulation from the thoracic crus of the diaphragm to the left iliopsoas muscle. Based on the postoperative notes following left nephrectomy performed 29 years ago, we suspected that the internal pancreatic fistula had resulted from the postoperative scar. Conservative management was performed. However, occlusion of the pancreatic fistula failed. Subsequently, she underwent pancreatic body tail spleen merger resection, and the pleural effusion disappeared.