A 55-year-old man was admitted to another hospital because of dry cough and dyspnea that rapidly worsened before admission. Chest computed tomography revealed a low-density mass that nearly obstructed the main pulmonary arterial trunk. Pulmonary thromboembolism was suspected and treated with catheter-directed thrombolysis therapy. Despite optimal thrombolysis and anticoagulant therapy, his symptoms persisted. He was referred for further examination and possible surgery for presumed pulmonary thromboembolism. The mass appeared more likely to be a tumor than a thrombus on careful analysis of the magnetic resonance imaging. At surgery, the anterior wall of the main pulmonary arterial trunk, the pulmonary valve, annulus, and the right ventricular outflow tract were all invaded by what was found to be a tumor and were resected under conventional cardiopulmonary bypass. The resected area was reconstructed with a 25-mm-diameter bioprosthetic valve and Xenomedica patch. Final pathological diagnosis was primary cardiac leiomyosarcoma involving the pulmonary valve. Postoperative course was uneventful, and he was discharged 11 days after surgery without adjuvant therapy because he refused it. Ten months later, the patient was well, but a chest X-ray revealed some coin lesions in the bilateral lung fields that were thought to be metastatic tumor.