There is a wide range of anatomical variations in the pulmonary vessels. Some of these variations may cause surgical morbidity during general thoracic surgery. We intended to perform a left upper lobectomy for a 73-year-old male with suspected left lung cancer (lt.U, S3, 20 × 20 mm, P0, E0, D0, PM0, N0, T1aN0M0, c-stage IA). Preoperatively, we did not recognize the existence of the common trunk of the left pulmonary vein. After the open thoracotomy, due to a severe intrathoracic adhesion, we omitted releasing the adhesion of the lower lobe. We did not exactly confirm the location of the inferior pulmonary vein (IPV). After cutting the interlobular pulmonary arterial branchies, we resected the superior pulmonary vein (SPV) using auto sutures. After division of the lung parenchyma and incomplete fissures using auto sutures, we intended to resect the upper bronchus. However, we could not find an IPV at the normal IPV site. A thin IPV was found to be returned to the peripheral site of the resected SPV. The SPV and IPV formed a common trunk at the normal site of the SPV. Although we considered reconstructing the resected common trunk, we finally made a decision of performing an incidental pneumonectomy in order to prevent any postoperative complications on the reconstruction of the IPV such as thrombus occlusion at the anastomosis site and venous return congestion. Retrospectively, we confirmed the preoperative images of the computed tomographic scanning, which showed a narrow IPV that returned to the peripheral site of the SPV. It is important to confirm both accurate locations of the SPV and IPV when performing a lobectomy.