A 68-year-old man presented with a chief complaint being a cough. Based on a bronchoscopic biopsy, it was diagnosed at a nearby clinic as an advanced left lung cancer, and he was referred to our hospital. Chest computed tomography (CT) scans demonstrated a giant mass of the left lower lobe, 14 × 12 cm in size, which appeared to have invaded the left atrium (LA). The operation was started with double vena cava cannulation via the right internal jugular vein and the right femoral vein as well as arterial cannulation via the right femoral artery. The patient underwent left pneumonectomy combined with LA resection using cardiopulmonary bypass (CPB), without aortic clamping, through left posterolateral thoracotomy under hypothermia (32 °C). The tumor-invaded LA was resected in a 3.5 × 3.0 cm area, with vascular clamping, and the stump was closed with 3-0 Prolene sutures. The surgical margin was free of tumor cells, and the duration of CPB was 28 minutes. The patient was smoothly weaned from CPB. His postoperative course was uneventful, and he received 2 courses of adjuvant chemotherapy. For a combined resection of the LA, it is safer to use CPB than simple vascular clamping, since the latter involves the risk of dislocation. If CPB is used, the tension of the LA is removed by blood extraction into the bypass, and bradycardia is induced by a reduction of body temperature, probably reducing the risk of clamp dislocation. Even when clamp dislocation or bleeding resulting from injury of the LA wall unfortunately takes place during surgery, these events can be dealt with appropriately during the use of CPB.