It is reported that myasthenia gravis (MG) with thymoma occupy 20% of all MG and extended thymectomy is recommended. After having operation, it is rare, but cases of recurrence of thymoma and, what is worse, thymic cancer from residual thymus tissue are reported. A 69-year-old man came to our hospital to have his dyspnea level examined. He had a past history of MG with thymoma and he had undergone extended thymectomy 17 years previously. Enhanced CT showed pericardial thickening and many tumors in the epicardium. Catheterization study showed dip and plateau pattern of left ventricular pressure. We therefore diagnosed constrictive pericarditis (CP). We performed pericardiectomy under cardiopulmonary bypass. He was discharged ambulatorily on postoperative day 24. Histological findings of the tumor and the pericardium showed that they were dissemination of thymic cancer. It was considered that thymic cancer caused CP and it was an extremely rare case. We think this is the first report to the best of our knowledge.